this being the release day of Mass Effect 3, I figured I'd do something small about what you might want to look for as you end your story. Hilariously, and by pure chance, this post coming out on time and on the actual day of Mass Effect 3's release means that it has gotten a more punctual response from me than any other real holiday thus far. Cute, no?
Answer The Questions
This is one of the most important parts of an ending: answering questions. As your story has gone on you have hinted and painted outlines of a great many things that are going on. In the end it is the time to answer those questions, reveal the full truth about those hints, and paint in the details of those outlines. You don't want your players - or audience if you are doing another type of story - scratching their head and going "wait, what the hell just happened? Why? How? When?" with a mixture of other/repeated question words.
Now, this doesn't mean that you have to answer all the questions, leave some room for further expansion ("save some of that for the sequel!" if you will,) but you want to answer the vast majority of them. This is true for a segmented story where you have planned "sequels" coming along, and is also true of the end arc or end of a single shot story. Answer the questions. If need be, hold a Q&A session with your players afterwards to explain what happened and what went into that happening. It can be kinda cool to see what the players were interested in.
More Of Whatever Made It Awesome
Let's be honest here, the end of something is our last chance to get a good fix of whatever fumes we were getting high off of. If your game ran on epic combats, you need to have an epic combat at the end. If you ran on witty dialogue and court intrigue, then have some of that saved for the end. The point is, whatever form of "action" your players came to expect and want from your game needs to be there in the ending. This may mean catering some parts of your end to meet certain characters' needs, but that is ok. As a GM/story teller also keep in mind that the end is your absolute last chance to give a deserving character a crowning moment of awesome. Don't deny them.
Old Familiar Faces
The end of a story is a great time to make some links back to the beginning. The old trainer, the old adviser, the old enemy, and the old friend can - and in many cases should - make an appearance. Making everything linked can bring the whole story together in a nice way. It can show how things done way back when helped to shape what is going on now. This is especially awesome in a table top RPG where what happened then is stuff the players may have forgotten about, or didn't think you had been paying attention to.
Last, for this post anyhow, the most important thing you need to do with an ending is to actually end it. I've seen a lot of games drag on because someone - GM or player - wasn't ready for things to end. I've seen lots of stories, and potentially good games, go on long past the point of being good into almost being a joke because they didn't end at the right time. Remember, the key to a good ending is knowing when to end, because everything will end eventually, so why not go out on a high note?
tl;dr for this section? Don't kill your own game's legacy and memorability by not knowing when to pull the plug.
There's more, I guarantee it, but those are the big ones for me. that said, hopefully ME3 will be the ending I'm hoping for. I've always been fond of the Mass Effect universe and all the stories it holds and could hold for the future. Seems a shame there's no table top RPG for it. Maybe Cortex Plus will grab it sooner rather than later? They've been knocking it out of the park lately with licensed games.