The first Modern Era is characterized by its crisp look and feel, as well as boss gauges sporting stars underneath instead of those ugly extra gauges, and that the POC is always available without focusing.

Unique Gameplay Mechanic: Faith System
To get anywhere point-wise in this game, keeping in convention with the idea behind the game's storyline, you must show faith in your ability.  Hitting/killing enemies and collecting items fills the Faith Gauge; the longer you can keep something on it, the more points everything is worth (which helps a lot when it comes to earning Extends).  If you're unable to kill anything or collect items for a bit and the gauge empties, you'll gradually start losing Faith Points, and if you die, you lose half your total Faith value (because dying shows a lack of belief in yourself)!

Unique Gameplay Mechanics: Spell Card Deck & Weather

Spell Card Deck
This is the first fighting game in the series to use the spell card deck system, which is also used in UNL.  Each character automatically draws from a deck of 20 cards (customizable for each character in your Profile in the Options screen); there are three kinds, signified by their border color:
Spell Cards perform the girl's Spell Card magic (in a style most fighter players would call a Super).
Skill Cards enhance or change the girls' directional-button moves for more power or versatility.
System Cards perform special functions, such as changing the weather or gaining immunity from attacks.

Weather System
Weather conditions (which are character-related in Story Mode, but are random in all multiplayer matches) each have a specific effect on gameplay; some enhance or restrict certain kinds of attacks, where others have more unusual effects, like making it harder to move in specific directions or changing the distance between the combatants randomly.  Knowing your attacks is always good, but knowing what changes the weather brings can help you secure your victory.

Unique Gameplay Mechanic: Special Abilities
In this game, not only is your shot type a shot type, it comes with a special ability borrowed from your youkai guide.

If you pick Reimu...
with Yukari: If you're close to the edge of the screen, your shots wrap around, and you can also warp from one side of the screen to the other by letting go of all non-movement buttons and double-tapping offscreen.  (Just make sure there aren't any bullets over there first!)
with Suika: You can auto-collect all onscreen items by letting go of all non-movement buttons.  Great for newbies!
with Aya: You gain a third ridiculously fast level of movement; let go of all non-movement buttons.

If you pick Marisa...
with Alice: You can stock up to 8.0 power rather than 4.0 (which is good, seeing that bombs cost power in this game).
with Patchouli: You can change the formation of your option orbs; press Fire & Focus simultaneously.  In cycle order, Fire formation is for forward focus, Water does area coverage, Wood takes care of weird angles, Metal fires sideways, and Earth fires backward.  If you know the stage (or boss) really well, you can annihilate everything in no time flat.
with Nitori: Your bomb is instead a defensive barrier - and you get reimbursed half the cost if it turns out you didn't need it.

Unique Gameplay Mechanic: UFO System
Certain enemies with UFO marks flying around their heads will drop little UFOs when defeated, which then randomly start floating around the screen; some change colors, some don't.  If you collect 3 in a row of the same color (or one of each), it summons a large UFO that starts absorbing onscreen items: if you saturate it to 100%, you get an additional prize on top of whatever you get for shooting it down!  Don't take too long shooting it down though, or it'll leave the screen, taking absorbed items with it.

Red UFO (For Extra Lives)
Saturate It: 1 Life Piece, Score Multiplier (1x or 2x)
Shoot It Down: 1 Life Piece, 1 Small Red UFO

Green UFO (For Bombs)
Saturate It: 1 full Spell Card, Score Mutliplier (2x)
Shoot It Down: 1 Spell Card Piece, 1 Small Green UFO

Blue UFO (For Points)
Saturate It: Score Multiplier (8x)
Shoot It Down: 1 Small Blue UFO

Flashing UFO (For Special Effects)
Saturate It: 1 Small Flashing UFO, Score Multiplier (4x)
Shoot It Down: 1 Small Flashing UFO, item packet's contents are reversed when returned (Power <-> Point)

Unique Gameplay Mechanic: Photography
As was "Shoot the Bullet," "Double Spoiler" is also a photography game.  All of the things listed in StB's unique gameplay mechanics section apply here as well, with the following additions:

Orientation Flip
In this game, you can flip the orientation of the frame back and forth between portrait and landscape anytime you want by hitting the [X] button.

Switching to Hatate
Once you clear the Spoiler 4 stage, you'll unlock Hatate, who comes complete with an intro stage (S-5) that teaches you how she's different from Aya.  To switch back and forth between Aya and Hatate on the scene select screen, hit [C].

The Differences Between Aya and Hatate
Unlocked her with the scorefile and can't be bothered with an intro stage?  Here are the major differences between our two spirited reporters' cameras:
Recharge Speed:  Hatate wins that one, hands down.  She charges at double Aya's speed.
Zoom Functionality: Aya is far more able to take pictures from a distance; Hatate's zoom sucks terribly.
Starting Orientation: Hatate starts in portrait, Aya starts in landscape.  (You can always hit Flip, of course.)
Frame Size: Aya takes standard 4:3 photos, Hatate takes panoramic pics.  These each have their own advantages and disadvantages depending who you're up against, though Aya can snap larger holes in bullet patterns.

The current era of Touhou games feature boss life gauges that wrap in a circle around the boss herself, so you don't have to glance at the top of the screen, and text that goes in callout boxes instead of an ADV at the bottom of the screen.

Unique Gameplay Mechanic: Bullet Freezing
The enemies shoot way too many bullets, don't they?  Yeah, they do.  Well, in this game, not only can you fire back, you can freeze enemy bullets.

What You Can Freeze and How It's Done
* You can freeze ordinary bullets (which is the vast majority of what you'll see).  Charge your Ice Barrier by holding the Freeze [Z] button, then let go; you'll envelop the immediate vicinity in ice.  Nearby bullets will freeze, which causes other nearby bullets to do the same, which, if done correctly, can cause chain-freezing the whole way up the screen.
* Ice Barrier Potency:  Your Ice Barrier's potency (the percentage above Cirno's head) charges slowly like Aya's camera; hitting enemies with your rapid-fire shots and any successful freezes from your last usage charge it faster.  The percentage determines how powerful the effect will be if you use it right then: using it at a full 100% can easily freeze the entire contents of the screen, whereas using it at 30% (the minimum charge it lets you use it at) is barely enough to afford you an extra moment to think.
* Use Perfect Freeze:  Cirno's spell card freezes all onscreen bullets (including fireballs; see below).  You charge Perfect Freeze energy by hitting enemies with your shots and successful freezes.

What You Can't Freeze and Why
* You can't freeze fireballs with your Ice Barrier - the only thing that can freeze those is Perfect Freeze.  You'll start seeing fireballs in stage 2, complete with a reminder that this is the case.
* You can't freeze lasers.  In fact, lasers will break any ice you've got going on, turning it into bullet "shards" that can hurt you.  Lasers will start showing up in stage 3.
* The Extra Stage's hazard is beams, which drain Cirno's Motivation gradually.

Where's my Life count?
Instead of displaying a straight-up number of lives, Cirno has a Motivation count; 100% is equal to one life, and the maximum is 1000% (or 10 lives).  Raise her motivation by hitting and freezing enemies.

I level up in this game?
There aren't any Power items, so yeah.  Cirno gains experience by defeating enemies and successfully freezing them; each time she levels up, she gains more firepower (icepower?), up to a maximum of Level 16.  (Who knew Cirno could count past 9?)  On Easy you start at Level 3, but on any difficulty above that, you start from Level 1.

Unique Gameplay Mechanics: Girls Came Prepared, Divine Spirits & Trance System

Girls Came Prepared
There's a reason there isn't a shot type select for any of our girls this time around - they brought everything they had.  Two shot types are active at all times; one weak one is always used whether focused or not (such as Reimu's non-homing amulets), and she switches automatically between two strong shots depending whether she's Focusing or not (for example, while unfocused, Reimu uses homing amulets, but when she focuses, she switches to her Evil-Sealing Needles).  All shot types gain power together (so there are no worries as to whether you're focusing when you collect items).

Divine Spirits
Along with power-up and point items are the little translucent orbs, which are the divine spirits in question.  They function similarly to UFOs, but they don't randomly drift about (something many players will be thankful for).  Each serves a particular function:
Blue Spirits add a bit to your score and your Trance Gauge.
Pink Spirits are this game's Life Pieces; collect enough and you earn an Extend.  Each subsequent life requires you to collect more pink spirits than the last one.
Green Spirits are this game's Bomb Pieces; collect 10 to earn an extra Spell Card.
Gray Spirits appear when you defeat a lot of enemies in a row, and also appear when you die. Their main purpose is to add to your Trance Gauge, though they also act like gold-bordered point items that are worth maximum points no matter where on the screen you collect them.

Bosses also drop spirits between phases along with the usual power/point item batch.  In addition, they drop blue spirits at all times - at a slow rate if you're far away, ridiculously fast if you're right next to them.  ("Boss grazing" in this fashion is used in score runs.)

The Trance Gauge
In the lower-left corner of the screen is the Trance Gauge, which starts the game at one full flame, and goes up to a maximum of 3 flames.  If you have at least one full flame on the gauge, you can enter Trance by hitting the appropriate button, which makes your girl invincible and uber-powerful for a limited time; how long depends how full the gauge was - a full gauge of 3 flames is worth 10 seconds maximum.

If you have at least one full flame on your Trance Gauge when you get hit by a bullet, and failed to deathbomb, you'll enter an automatic death-Trance that lasts however long you had toward your 10 seconds.

Regardless of how you entered your Trance, the gauge will be completely empty afterward.

Youmu's Sword
Youmu functions differently from the other three girls; when she Focuses, instead of switching to a different shot type, she stops firing to charge a usage of her swords; let go of the Focus button to unleash a combo of slashes.  The combo starts out at 3 slashes, and gains an extra slash per power level (so at the max of 4.0 power, she slashes 6 times).