I just beat the game for my first time (I got Iorveth's Path,Saved Triss, I let Sile go, and I killed Letho) and I was very disappointed with the end.I mean probably doesn't matter what you do there is no cutscene or anything after the game is over.It's kinda hard to explain what I'm trying to say but is like, they dont say what happened with all the other characters in the game (no even what happened with Gerald and Triss).I dont know, they just should put something after all.The game pretty much does not have an ending.
And is there any difference between those 16 endings?Or the only difference is that different people died in each ending?

I think what they mean to say when they said 16 endings, was 16 possible beginnings to The Witcher 3.

Well, for all intends and purposes the story of the 'Assassins of Kings' was concluded one way or another, so Gerald leaves Loc Muine and moves on. Where to ? We'll probably know in the Witcher 3. There are obviously unfinished stories and loose ends that need to be tied up. It is a sudden ending, no doubt about it, but it does leave things to look forward to. As for your companions, the journal does provide some vague explanation regarding their location after Chapter 2 but I personally believe, it was intentional to leave the world in one of those 16 'states' (wouldn't say endings, since the differences are subtle and we don't see the full impact of our choices). It's more of a 'to be continued' ending, so to speak.