Can continue to play after the ending? Or do we have to save before the final area?

Does the side mission storyline have a effect on the ending? Or If do the side mission storyline appear in the ending if I completed it?

Yes, you can continue playing.

There is no ending slide, which irritated THE HELL out of a huge portion of the fanbase.

This pretty much annoyed me, because i would have had to finnish every quest before i did the main questline. I had a brief conversation about it on twitter with William Strife of the Yogscast about two months ago. He tweeted that there was going to be no level limit in game, so i tweeted back "A no level limit is good, being able to do quests you missed would be better." I was insinuating that if you didn't have Broken Steel DLC installed, you'd have to start again from the beginning. glad they've sorted it out in Fallout 4.