http://sims.wikia.com/wiki/Expansion_pack#The_Sims_3Almost all expansions include new professions, new "life states", stuff to build, etc, so I'm skipping those. so, "major" changes would be:
World Adventure adds:
1) traveling
2) quests
3) tomb raiding (needs to babysit the raider constantly tho, becomes almost a 3rd person puzzle/rpgish game during those periods).
4) Tiberium (yes,
that one).
Ambitions adds:
0) not much
1) "roleplaying" professions (5 of them): basically, jobs that you have to babysit to actually advance. so you have to actually go out and put out fires, bust ghosts, redesign a house, cut people hairs, etc. I actually built the Ghostbusters fire station very faithfully, based my floorplans on the Ghostbusters: The Game game
too bad you can't "team bust" Ghosts, so having 3-4 paranormal investigators in a house is a PAIN IN THE ASS since each get their own emergencies to go alone, simultaneously.
2) ... pretty much that's it. chances are you'll only have one family member in one profession if you want to keep your sanity.
Late Night adds:
1) clubs
2) highrises (multiple people live in them but I'm not sure it has multiple floors. I mean, some do have 2 or 3 floors but it's not like a 40 story building has 40 available floors. mostly it's cosmetic. i think you pretty much only live in the penthouse, and a building may have two clubs: one basement and one in penthouse). also elevators can be annoying.
3) vampires (meh?)
4) multi-purpose civic buildings, I think. maybe it was a previous expansion.
5) star fame thingy (more annoying than useful... you can't even avoid gaining fame just by accidentally talking to someone famous. eventually EVERYONE in town will be famous since they talk to each other).
6) form a music band
Generations adds:
1) kid stuff, I think? Dreaming, slumber parties. Not a lot of gameplay changes from what I've seen.
2) boarding schools that you pay for
Pets adds:
1) pets
2) horses (see #1...)
3) parrots? (see #1...)
4) hamsters? (notice a pattern)
Showtime adds:
1) no idea
2) ok, apparently you can become Cher or the Siegried Bros or whatever.