Halo 2 was supposed to have 4 player co-op, 3 more campaign levels, and Forge. All had to be cut to meet the release date.
Halo 3 had 75% of its planned Forge features cut and sold later as DLC.
The only reason these games seemed "complete" is because you never got to peak behind the curtain until years afterwards. There's a huge amount of transparency and communication with game developers today and it leads people to erroneously assume that there was some "golden age" of game dev where a game wasn't released until it was "ready", which is complete bullshit. Gamedev has always largely been a play it by ear undertaking.
I am just tired of this endless hand wringing. We've played the game at this point. It plays great. We've seen the basic structure of the campaign and it looks fun, even if the demo from a year ago didn't have the best graphics, it's clear they know what they're doing when it comes to map design and mission structure. Are people expecting them to show like an hour long campaign demo before release or something? I honestly don't think anything will satisfy some people.
None of those games had regressions though. Once Bungie added a feature it was never taken away in subsequent releases. Halo 2 dropped Race Mode, but other than that I can't think of much.
Infinite will be the first game in the franchise without coop at launch, and the first game since Halo 3 to not have Forge at launch.