They found that, with point shooting, the 1911 shot a little low for most people. They changed the mainspring housing from flat to arched, which made the gun point higher when you held it.
They found that some folks had trouble reaching the trigger. They changed the length of the trigger, making it shorter, and they machined little notches into the frame, on both sides, behind the trigger, so short fingers could get in easier.
They found that the hammer spur would often pinch a layer of flesh between the back of the spur and the top of the grip safety, so they shortened the spur slightly and lengthened the grip safety, slightly.
The biggest VISIBLE difference is the notches behind the trigger guard. Everything else is replaceable. You can put a 1911 trigger on an A1, and vice-versa. Same with flat and arched mainspring housings, hammers and grip safeties.
The 1911 rear sight had a very tiny 'U' shaped notch, and the front sight was narrower than the 1911A1 (not sure of actual dimensions, but fairly subtle).
The 1911A1 (the 'A1' in military speak in those days meant 'alteration one') had a subtly wider front sight and was squared flat on top, rather than almost a point, and the rear sight notch was slightly wider and deeper and was square on the bottom.
However, one does occasionally find a G. I. pistol with a 1911A1 frame and a 1911 slide with the original sights. These are presumed to be arsenal rebuilds.
I believe it was at the 1911A1 introduction the pistols went from a polished blue finish to the more familiar, more durable and cheaper parkerized finish. The finish change may predate the 1911A1; no doubt one of our more studied brethren will correct me.