A list of reasons that movies based on games suck:
Every medium has difficulties in the adaptation and transition from one form to another. Books that are turned into movies tend to be too heavy, suffering from crushing 500 pages of plot into 120 minutes. Comic book movies betray the worst of their source material: the silliness of outlandishly-costumed vigilantes Fighting Evil while mishandling their soap-operatic personal lives. Nowadays we even have a whole genre of movies adapted from other movies, particularly in the case of foreign flicks (read: Japanese horror). Because sadly, Americans may run screaming from the theater if they have to watch anything without a cracker as a main character, or listen to the devil-tongues of furriners.
I watched Advent Children the same weekend that I finished the third Prince of Persia game. AC is blatant fan-service: Square managed to squeeze every character and reference into the movie as was humanly possible. A friend and I had a good time calling out various references ("Ha! Limit Break!" "Bahamut!") as it went along, but that's not the same as saying that it's a good movie. It just means that it's not a terrible two hours if you are a tremendous dork. Even if you are a tremendous dork, you probably need to have played the game--and since I never actually got more than an hour into FFVII, I was lost pretty much the whole time.
The shame of this is that Final Fantasy VII is one of those games that (for some reason) brings back misty-eyed reminiscences for people who actually have finished it. You would think that just remaking the cinematics from the game using new tech, and finding a way to glue them together narratively, you'd have a half-decent movie. With games being more cinematic these days, it would make sense to do that, right? Take GTA's non-interactive sequences, play them sequentially, and you've basically got a Godfather knockoff right there.
Instead, when I look at the list, I see two different approaches: one is to overcomplicate an otherwise faithful script, and the other is to pretty much disregard the source material altogether. The former isn't hard to understand, since (despite my enthusiasm) much of what glues a game together is repetitive and relatively uninteresting to watch. Nobody wants to watch someone level up after killing ten rats, and you can't base an entire movie on drive-by shootings (now that I've said that, someone will try). Leaving those mechanics in the movie (read: Doom) is a boneheaded move. The key would seem to be striking a balance between faithfulness and the rules of cinema--finding devices to fill the space where the game took place, and using them to build character or flesh out the already-existing plot--more "based on" and less "inspired by."
There are people who would say that you can't make a good movie from a video game by definition, because they're basically trash. I actually take some heat from friends who will read here every now and then, because I occassionally discuss the games I play at a higher level than they might deserve. To those people, all I can say is "Pirates of the Caribbean." Was there ever a dumber idea than to make a movie from a (let's be honest) lame and uninteresting theme park ride? Cory Doctorow might get all slobbery when someone proposes turning Disney animatronics into a feature, but I'm pretty sure the rest of us heard the news a few years back and rolled our eyes. Now look at it (and try to ignore The Haunted Mansion while you're there).
I'd like to discuss pulp at a later time, but in short: there are many great movies that are based on material as shallow as the average video game. Spiderman turned out pretty well. Seven is nothing more than a cheap thriller that realizes the best of the medium (great acting, excellent plotting, and a good gimmick). When it all comes down to it, classics like The Maltese Falcon are based on dime-store material. I'd like to say that there are a lot of games that would meet some level of quality--maybe not enough to rise to greatness and widespread acclaim, but competence would be nice. I don't think it's too much to ask that they be as good as, say, Mission Impossible.
Ah, easy for me to say. At the center of my optimism is the belief (new, I'm sure, to a generation of people who grew up post-NES) that a game can be just as much a cultural artifact as a book or a film. The implication of many conversions is a lack of respect for the original--Silent Hill is one of the first that seemed to take itself seriously, for what good it did.
Could just be me, but I'd watch that.