For my game, it wasn't so much that I hated it before I played so much as I played way too little of it and never really gave it a chance.
The game was Shadow of the Colossus, a game I played at a place that I could only describe as a "new arcade," a large collection of HDTVs and consoles with an enormous library of games you chose from. Rather than using a token system, you payed by the hour.
Needless to say, this lead to me rushing into Shadow to see the gameplay, which isn't what the game was about, so I ended up missing out. The first boss turned out to seem way too tedious to waste all my time on (I thought to use arrows only to find out they were didn't do much damage).
Later, I looked up how you would go about beating the boss, just to see if I was right about it being nothing more than a long ass long-range fight. When I found out you had to attack his leg and climb up, I decided I would pick up a cheap copy of the game and at least beat the bastard and see if things got better (bit of pride on my part).
Turns out I was completely wrong about the game and have clocked over 100 hours (don't know exactly how much, but over 10 playthroughs, the first the longest and the shortest being somewhere between 6-8 hours).
Otherwise, I was pleasantly surprised by the following games:
-Demon's Souls
-.hack//GU
-Castlevania Lords of Shadow
-RE5 (though not that much with it, but it wasn't AWFUL)
-F.E.A.R.
-Crysis 2
-CoD: Black Ops (much more fun than I thought it would be. Still glad I only played a roommate's copy though, as I doubt I'd replay it before other shooters)
-The Darkness (if you can get past a few problems, such as Jackie's mouth, learning to properly use crawling dark and minions, and the lightning part, you've got a great game here)
On the opposite side, there were a few games I got let down by
-Half-Life 2 (this is your fault internet. I got to play this around the time the Orange Box finally hit bargain bins. This game had been overhyped to hell and back)
-inFAMOUS (not bad mind you, but I played this after Prototype so Cole felt significantly underpowered
-Bethesda games (especially the Elder Scrolls games. Fallout 3 turned out to be much more fun when I took a recommendation to use melee weapons though...)
-Grand Theft Auto 4 (I truly hate this game. Not only was it nearly as hyped as HL2 when I got to play it, but it really wasn't that much fun for me)
-Borderlands (just not as "wacky" as I thought it would be. Not it's fault consider the "For Real Gamers commercial wasn't Gearbox produced)
-CoD: Modern Warfare 2 (Ugh, this one wasn't as big a let down due to the internet reaction, but to sum up this properly, it is the reason Black Ops surprised me for the better)