The ending doesn't really change, extended cut just makes it easier to achieve the "best" ending (highest points needed) by lowering the requirements so you no longer
have to play online to get it. Add DLCs on top of that and it's very easy to get enough points. Extended cut also adds a few extra cut-scenes, but nothing major. Leviathan is IMO a more important DLC, because it adds a some insight into Reavers.
And yeah,
@Redlambs. Like I've said before, ME1 is a RPG with FPS elements, whereas 2 and 3 are more of a FPS with RPG elements. I love all three games though, but 1 will always be my favourite. I'm assuming 4 will follow the FPS trend though, because it's obviously very popular.