There is a reoccurring theme in modern video games these days: the endings suck. Endings can suck for several reasons, so I'll list a few of them below.
1. The developers want to get as much money as they can so they leave an ending completely open, concluding only the trivial pursuits of the protagonist rather than the actually story worth problem. Greed and gluttony swell up in these story writers like a hungry Parana in a microwave until they burst all over the script, covering it in their molten money-leeching slag.
2. Somewhere along the line, the writers felt cheated like a boy being taken to the dentist instead of the petting zoo like mommy promised, so they generate the biggest proverbial "SCREW YOU" they can and then flip off their audience with every middle appendage they can find. These writers, by definition, are trolls and simply either have nothing better to do, or feel justified in hurting people because of their imagined slights. Thor referenced.
3. There is the occasional writer who thinks it would be important to have a tragic ending because they think the audience will care more if the hero dies in a flaming bus while trying to save his or her daughter's pet cat, Snowballs. What they end up doing, however, is drag you along a brooding story while hinting that the protagonist is going to die by self sacrifice; because heroes can't die any other way. So when it comes time to bite the bullet and die in some heroic, sacrificial way, it turns out that Snowballs was never on the bus and is, in fact, sitting at home, curled up on your sofa, enjoying the ironic warm of the fireplace after having cough up a hairball on your favorite bed sheets.
4. Heroes become villains. Generally this rule is a stupid one and is often done in such a melodramatic way that it kills off the last glimmer of hope gamers had for a sound conclusion. The producers obviously thought "well, people hate cliched endings. I know! Let's do the opposite! Let's make the hero a villain at the end." So while you spend the entire game thinking you're either going to save the day or die trying, the writers are all secretly watching you from the shadows, laughing maniacally as you approach the inevitable conclusion of become one with the Dark Side of the Force. And then they justify it by saying "Well, even if they hate the ending, they're still going to enjoying having all these dark powers and going all bad A. for roughly three minutes before we roll the credits."
5. My least favorite: abrupt endings. Some writers live under the notion that by suddenly cutting the story off creates a good cliffhanger and therefore has a better chance of selling. They also use this style for the "make your own" endings. The hero has battled his or her way up the mountain, slayed the dragon, married the princess, and suddenly the evil wizard's eyes snap back open and he lunges at the hero with his magical staff and then the screen goes black. OR the hero gets inches away from achieving revenge (because face it, if you aren't saving something, you're avenging it) - something he or she has been plotting through the entire game - only to have all their hard work and planning amount to thirty seconds of cutscenes and a quick "press A NOW!" and then the bad guy dies and the closure is the protagonist staring off at the sunset, trying to be bad A.
That's my rant for now. There are more examples, of course, but I don't care. I could make a far better game if I had the means. You'd think these big developers would bother to hire a writer who actually knew what he or she or they were doing.
IMPORTANT NOTE: These endings listed above can and have been done well, but let's face it: writers who fall back on these tropes rather than make them work with their plot suck.