Oh my god. When I read your question, one game popped into my mind.
OBLIVION.
I was so hyped for that game. I had spent months hearing nothing but greatness about it. When I finally got it, I popped it into my disk drive and hopped right into it. At first I didn't know what to think. It wasn't bad... but I didn't love it either. As I played it more, I realized something. This isn't the game they promised me! I was hoping for a huge, open world RPG hearkening back to the days of Fallout and other classics in which I could be whoever I wanted to be. What I got was basically Grand Theft Equine.
It's not bad in the least, and has some moments of sheer brilliance (like when you do actual role playing), but (as Yahtzee said) everytime I started to feel immersed in the game world, some NPC starts humping the geometry or engages in an IQ-lowering conversation with another NPC. I soon got bored of the game and shelved it until I started getting into the business of modding. After that, it finally started to live up to its potential. All I can say for people who want to buy the game is GET THE PC VERSION.
Thank God Bethesda learned from their mistakes with Fallout 3.
Another game that had this effect on me was Spore, though 10 times as hard. I literally expected an experience that woul be nothing like anything other and would be the end to gaming as we knew it. Boy oh boy, was I wrong.
Coming back to it, it's a ton of fun, just not the earth shattering game that everyone expected. Besides, the content editors were so unreasonably awesome that they justify the full price of the game and its expansions. So, it arguably realized its potential more than Oblivion.
/wall of text