Really, the only thing that stalls me on a game is boredom--if I just don't find it interesting. And usually that happens because the game is constructed in a way that causes a lot of fits and starts and doesn't allow the space for the story & gameplay to develop. Most often this is due to timed puzzles and death sequences, especially the kind that results in a spectacular "failure" sequence that you have to watch over and over, a la Necronomicon. But lack of smoothness, especially in the beginning, can do it. Never could get involved in Voyage for that reason.
I do finish most things, though. There have only been a few I've disliked enough to abandon: Traitor's Gate, because I couldn't take a step without violating some security thing and it just got old (also I didn't think it made sense--if you're sending someone on this vital secret mission, why supply him with such substandard equipment, like a lockpick that only works on obscure 15th century padlocks?); Thorgal's Quest, because it just wasn't an adventure game, Frankenstien TTEOM: bad navigation and too much wandering around, and Voyage (BORING!).
"So then he says it's enough to take aim! What's that supposed to mean?!?"