WoW.
I'm still a fanboy, mind you. No other game, par Half Life 2, created such an immersive gaming universe. The beautifully detailed environments, the górgeous music, the characters, the story-lines, the humour, everything together creates a fantastic place to be.
However, 3 months into WotLK I just ran out of content. No matter how beautiful Grizzly Hills is in all it's aspects, when all you can do is run around and...well, do nothing, the fun kinda stops.
I might've stayed if my guild didn't screw itself up. One day I found myself guildless, I was kicked because they went all hardcore and didn't think a Friend's rank (I played WotLK casually, best time I ever had in that game) would fit with a hardcore guild. Safe to say that half of them didn't like the sudden change in mentality (most people played for fun) and me getting kicked (even though I had a Friend's rank, I was an age-old, well-respected member) and thus the guild disbanded.
So in the end, I had no people to play with anymore, I had no content to play with anymore and all I was left with was a gorgeous painting to walk around in with gorgeous music costing me 13 euro's a month. Sorry Blizzard, no matter how brilliant Azeroth is, I'm not paying 13 euro's a month to just look at stuff and listen to stuff. I can download that music for free you know, and wallpapers were invented for a reason. And that was a year ago.
On a non-WoW note, I also stopped playing GTA4, Assassin's Creed and Borderlands out of sheer boredom. All those games are just too damned repetitive.
I'm also having a hard time booting up Dead Space, simply because it's not scaring me. If a horror game simply isn't scary, you're usually left with a mediocre action game. I don't think the game is bad, mind you, it's trying really really well to scare me with those really good lightning and sound effects, but I'm just not scared by them. Sorry EA.
