Dragon Age Origins. The game took forever to suck me in past the origin parts, but after I got to the Magi Tower, things picked up. Unfortunately, after I left, it slowed down to yet another game-ending crawl.
Oblivion was another. I played through the main quest and did the Dark Brotherhood ones, and then I said "Well, that was fun." and gave up on it.
Until I discovered MODS!
Also, WoW. After 6 years of being off, the Cataclysm has grabbed me by the short and curlies, and said "Your gonna play with us... Forever and ever..."