I think Yahtzee has scored a solid hit on the issue, but there are other elements I'd consider.
We take different things for granted in movies and in video games. We take things for granted in each, certainly; each has certain tropes that we give a nod to and move on. A few examples:
If a character in a movie visited the same store eight or nine times, chatting with the amiable shopkeep each time, you, the viewer, would probably assume that that location and/or merchant would turn out to have some greater significance later in the movie. You would expect a shoot-out to turn the order and seeming domestic bliss of that store into a warzone, or you would expect the shopkeeper to offer a shoulder to cry on and some folksy, perfect advice at some turning point in the movie, or that the troubled missing ten-year-old would run to that store when they felt they had no place else to go.
In a video game, you probably kept visiting that shop (and hearing the same canned quip over and over) because it was necessary to get the gear to progress and served the designers as a beat to offset the action in the rest of the game.
Similarly, if someone kills an innocent bystander in a movie, it would be a huge turning point. You could expect them to spend the rest of the film wracked with guilt, or that it would reveal the character we thought to be benevolent to be far more sinister than they first appeared. Whereas if someone kills an innocent bystander in a video game, it's as likely as not a failure to align the aiming reticule correctly, or some sloppiness (intentional or not) on the part of the team-mate AI. You might give some zenny to the beggar outside of town to put your karma meter back in the blue.
A flashback in a movie is probably trying to tell you something about a character's history; a flashback in a game may be doing the same, or it may simply be putting the character back in an "easier time" so they can learn the lock-on mechanics against a scarecrow, and never reference anything plot-related again.
In video games, of course, we're hardly surprised when someone takes ten bullets in the torso and is sprinting down the beach five minutes later. In a movie, they better be a cyborg from the future if they pull that stunt, or at least reveal that they're wearing kevlar.
In a movie, we might expect a scene of characters doing important but technical, time-consuming, or dry activities (research, computer hacking, safe cracking, reading...) to be glossed over with a montage of cross-fades and maybe some voice-over and/or a cue on the score suggesting urgency. In a game, you might do the same with a minigame... Or in the case of reading (as witnessed by games like the Elder Scrolls series) you might just be expected to actually read the thing. (Go ahead. The plot will wait for you.)
Movies based on games are often hard-pressed to remind the audience of the material they're adapting, and it sometimes feels that they forget differences like this in the process. They expand on things players had no interest in knowing more about, or in ways that run counter to the expectations of long-time players. They stop and explain things with an embarrassment that highlights those things should have been cut out in the first place. They talk when we want to see more running and jumping and shooting, and run and jump and shoot when we actually want to hear more of what a familiar character would actually say in that situation. They wander between "cinematic reality" and "video game reality" to the point that we turn off our ability to be surprised, and with it, our ability to care about the proceedings.
Games based on movies often seem like they feel all the heavy lifting viz. plot and character has been done for them, so all that remains is to paste on some semi-familiar mechanics and call it a day. Rare is the game that wanders far from the plot, or offers a chance to move well-known characters in a totally different direction; even "prequels" or "side stories" often ape the action of their "inspirations" well beyond the point of "tribute". I think this actually happened more in the days of text adventures, where games like Tellarium's, based on popular works of fantasy and science fiction, could allow the players to explore familiar worlds in ways the original authors never did without breaking the game's development budget. Today, licensing is a big business, and most developers aren't going to do anything that might put off the most incurious fan of the franchise, assuming the property's oversight team would even allow them.
