I don't know, I personally never really got a chance to see the original ME3 ending. Of course, I also haven't really gone out of my way to hunt it down, because I just wanted to finish MY story.
if all you've seen is the "extended ending", then they basically cut a few scenes.
First you don't get to choose to "do nothing" (if you ever done it) to "lose the game". You can only choose Red or Blue... or Green with high enough galactic score(? forgot what it's called)
Oh before that, your companions just "disappear" from the storyline. Nothing to tell them to go back to the ship, it's an order, etc.
After you activate the Catalyst there's no scene reminiscing of other companions/lovers. You just see Normandy escaping the blast, and land on that planet. Then Joker walks out with/without EDI. and that's it. IIRC.
Oh right, I made a video then. Here it is:
That is the worst possible ending btw, lowest score, making the worst decisions, with no remaining ME2 crew alive, etc.
huh... I didn't upload the "Part 26"... ugh. That was basically the ending ending...
well here's all the choices after you pick your poison:
and that was it. No extra scenes, no explanation on what happened, no knowing what happened to other species/planets/etc.
I have a bad feeling about it as well. Feels just like Dragon Age: Inquisition. It was a great game at launch, but I couldn't bring myself to finish it. It was just too much and didn't keep me interested. My hopes for this game aren't really high, but I hope it shows me otherwise.
I never finished Inquisition, because halfway through after finding that Castle, my game got corrupted, so I had to restart, and every time Solas tried to show my way there, it would crash, so I just gave up. Probably a bad mod I installed, and didn't bother continuing because the save file itself was corrupt.
Other than that, I enjoyed the game quite a bit.