Oh wow, been a while since I've seen you around.
That's such a broad question it'd be tough to even start to answer it completely.
I suppose the biggest mechanical change would be the Buffered DACUS, which can be done out of SHDLs, Downthrow, and even shield drops and getup attacks for quick/surprise kills. The most common application of this would be Dthrow -> BDACUS, because if your opponent DIs down/away(like they're supposed to) they get hit, if they airdodge and you charge it they get hit, and if they DI up to avoid it you can just kill with a Bair.
Dunno if you were around for the whole "jab x2 is godly" realization, but well, it is, and you can even go as far as to cancel it into itself multiple times. There are also things that I think are common sense, but useful nonetheless, like reversing a firebird so the slight backwards kick at the peak will push you back onstage so as to not get the hell edgeguarded out of you.
There are also the things that help the matchup vs Falco that I'm not sure you were around for, like being able to SDI +tech Falco's CG -> Spike.
And then there's some random gimmicky stuff that was discovered which is more fun/interesting than it is useful, like Foolstool -> Laserlock combos and Ledge Hopped Triple Lasers and the like. Stuff you'll never use in an actual match.
I know there are things I'm leaving out, and I may have listed things you already knew (it's tough to remember when some of this stuff was first discovered). It should be a good chunk to start with though.