Figured with some of the really awesome games I've played lately, it'd be neat to start listing game design elements that make a game that much more interesting and fun to play. I'll start~
Body part targeting systems - allowing the player to manually select what areas of an enemy to target works in a great pen and paper type of system in our modern gaming setup; VATS in Fallout 3 and Vagrant Story's "sphere selection.
Active Reload - giving a player three opportunities in one single movement to reload their weapon during combat, and rewarding them for matching up the timing; full active, main active, or just letting the gun reload over the full time, and giving them a gun-jam setup if they attempt and fail which adds to the rigors of combat
Cover System / Blind Fire - Gears is a great example of this setup, and while it can be annoying if the environment isn't set up properly, it can lend to some pretty cool tactical situations for the player to work to their advantage, even in a rather one-sided conflict.
Replies
Localized Damage. And thus the headshot was invented. Pushed to incredible heights by Soldier of Fortune's GHOUL system, allowing the user to shoot off any limb.
Public Quests. An open RPG quest system that rewards anyone for passing by and helping out and giving great rewards to groups working together for a common goal mixing cooperative gameplay with competitive. Thank you Warhammer.
Strong use of silhouettes to define enemies and level of threat.
Never under estimate humor in all its forms and at unexpected times.
Well thought out lighting and levels.
Puzzles that don't seem like puzzles, but are built into the game. Thinking about Bioshock and the hacking puzzles, lock picking in Fallout3, climbing in Assassins Creed.
Stealth, mixed with great AI is always a treat.
Fighting systems that are complex but easy to use. Materia in FF7, sockets in WoW.
Customization is always welcome.
Recharging health - Like GoW and others, I really like that I don't have to go hunting for health packs all the time; it seems to keep the pace up. Not for all genres, of course.
... having a hard time thinking of more, some of my favorites have already been mentioned
Crawling while downed in Gears2. Deepens the experience of being bait.
PP in Dead Rising. It gave using the camera purpose.
Reversing Time in Braid. Good times.
Randomized Environments in Diablo 2. Longevity much?
Molotov Cocktails in Left 4 Dead. For your downed partners.
Brush painting in okami
Narrative told through the environment.
Properly Queueing up abilities in traditional MMORPG's - City of Heroes.
Cooking grenades - Many FPS games: Makes the use of grenades much more specialized, tactical and interesting. Also adds the possibility of blowing yourself up if you take too long.
(Deus Ex, System Shock, Vampire Bloodlines)
Open world is neat but often sits at odds with story progression. Multiple controlled paths depending on playstyle allows the best of both worlds, I think.
Hybrid Health Recovery.
(Resistance)
I hate the "hide in a corner to regain health" stuff in games. It removes consequences for stupid actions. Resistance's is nice because it's a fun compromise between traditional health packs and the dumbed dow- sorry, "streamlined" - hide and heal console FPS method. I think Riddick's might be similar, too?
Well Placed Mini-Games.
(Fallout 3, Gears of War)
I've been really liking the lockpicking minigame in Fallout 3 and active reload in Gears of War... even the little keypad game in The Cassandra Project Deus Ex mod was a nice touch. When done well they're a fun change of pace and an extra wee challenge. Even stuff as simple as the directional finishing blow in No More Heroes worked well.
Quick-Select Conversations
(Dreamfall, Mass Effect)
This sort of thing really helps in-game conversations flow more naturally and keeps the player immersed and doesn't halt the gameplay during important dialogue.
Character Customisation
(Deus Ex, Saint's Row 2, Tony Hawk 8, LittleBigPlanet, loads of stuff...)
I am a sucker for character customisation, especially if there's an RPG element involved where you can improve certain stats of your character. But ultimately all customisation is welcome and lets people attach themselves to their character far more than usual.
FPS-Only Camera
(Half-Life series, Call of Cthulhu, Crysis, Mirror's Edge)
Again, it's an immersion thing. Seeing everything happen from a first person perspective really helps you believe you're there. There's not a lot of excuse for pre-rendered cutscenes these days, especially when you can keep it all in FPS camera and see your arms and legs actually there in the world. Crysis does like to steal control of the player for cutscenes, though. That's bad.
L4D has really affirmed the fact that I don't always enjoy shooting other people (in halo, CoD4, TF2, Unreal etc). And that I actually like to run through a game with a team or friend. One of my biggest disappointments in CoD4 was no coop play.
Since I'm also hooked on how good L4D is, I like their reload system.
Where you press the reload button but still have the opportunity to shove zombies aside. Takes the whole "wait for reload" aspect of shooters away cause I always feel like I can do something.
Personally I hate unlocks in competitive games, they inherently unbalance the game and feel like a really cheap way to keep you playing.
Perks on the other hand are nice, as others have mentioned customization elements are satisfying.
I quite like the melee system in cod4, it's a bit overpowered but its so fun
first thing that comes to my mind is Okami.. where you can actually get currency and useful items by having correct timing. something to do during loads! i'll update this with more later, but awesome topic idea.