Oh yeah! Thanks xD

Defense Grid is awesome too! Is it still free?

Nope.

Bollocks. Must have just been a short promotion :( Still worth it though!

I have thoroughly enjoyed: Limbo and Braid.

There is a new one Bridge that looks really good too. All of these have Demo's you can try. Braid gets really hard in my opinion.

N+
Castle Crashers
Torchlight
Geometry Wars
Geometry Wars 2
Kingdom of Keflings
World of Keflings
Banjo Kazooie
Banjo Tooie
R-Type Dimensions
Doritos Crash Course 1 and 2
From Dust
Outland
Limbo
Peggle
Pinball FX2
Portal Still Alive
Rocket Knight
South Park Tower Defense
Defense Grid
Fancy Pants Adventures
Trine 2
Call of Juarez Gunslinger
Serious Sam First, Second, and Third
Counter Strike Go
Sine Mora
Toy Soldiers and Toy Soldiers Cold War
Ikaruga
Rez HD
Shank 1 and 2
Scott Pilgrim
Minecraft
Alan Wake American nightmare
Warp
Burnout Crash
Death Spank games
Insanely Twisted Shadow Planet
Dishwasher games
Monkey Island games
The Maw
Worms games
Dungeon Defenders
Dead Block
PvZ
Joe Danger games
Hydro Thunder
Costume Quest
3 on 3 NHL Arcade
Raskulls
Comic Jumper
Misadventures of PB Winterbottom
Jetpac Refuelled
Rock Band Blitz
Perfect Dark
Call of Duty Classic

You assume people have friends. 

My best friend who game shares on my account doesn't even like playing good games that I recommend.  No chance that he'd play Harmony of Despair.  My other friends don't own the game.  So I haven't played it with friends.

So far good blend of genre... Good thing is you get demos with every game...
I forgot orcs must die... Crimson alliance... Lara croft:GoL.... If you have Kinect wreckateer is fun fruit ninja... Panzer general allied assault, kind of risk meet close combat(RTS) game... Dungeon defenders, brain challenge, bomberman live with friend can be fun... Harms way is free, risk faction (risk game tone of fun in MP even local MP, did some after bar party nights with buddies till 7am with that one), castlevania: SOTN
those are couple addition...
if you haven't played arcade games this Gen yet... Trust me you don't need to jump in Gen 8 till 2015 at least

and don't trust that guy that said you get what you pay for... Among those game some completely destroy supposedly AAA retail title... I'd pay gladly 60 bucks for some of them over a COD or a BF.. And I love FPS

This

http://marketplace.xbox.com/en-US/Product/Bejeweled-Blitz-LIVE/66acd000-77fe-1000-9115-d80258410a57

it has more Gems than you can count :D

I'm only recommending those that I've actually played.

Recent good games (2013):

CastleStorm
Far Cry 3 Blood Dragon
State of Decay (which you already mentioned)
Mickey Mouse: Castle of Illusion
Skulls of the Shogun

Classics:

Shadow Complex
Fez
Minecraft
The Walking Dead
Portal
Trials HD
Trials Evolution
Mark of The Ninja
(Ms.) Splosion Man
Bastion
Peggle
Limbo
Plants vs. Zombies
Lara Croft and the Guardian of Light
Castle Crashers
Battlefield 1943

Other:

Deadlight
Bejeweled 3
Super Meat Boy
Anomaly Warzone Earth
Defense Grid
A World of Keflings
Puzzle Quest (1&2)
Trine 2
Costume Quest
ilomilo
Torchlight
Dead Rising Case (Zero & West)
BattleBlock Theater

As a side note, Brothers A Tale of Two Sons got great reviews from both reviewers and gamers, but I was personally extremely disappointed. The puzzles are extremely simplistic, the platforming unchallenging and just there to make you progress slower, it's glitchy (had to replay whole levels because it wouldn't let me finish them). On top of that while the controls are interesting from a gameplay point of view, they are just extremely confusing and simply won't ever work fluently.