The best thing that really happened to ZSS was the hitstun, and it's really all she needed. ZSS's biggest weaknesses in vBrawl were the fact that she had a mediocre approach, and that her average damage output per "combo" did not make up for it. In Brawl+, last time I played, both of those problems were solved. ZSS's short hop was shorter, and her fall speed was higher, making her nair and uair much more potent offensive threats, and both of them led into massive strings of powerful combos involving uair, fair, Plasma Wire, dash attack, jab, dtilt, and the list goes on.
Her defensive game also improved. Narrower PS window makes Plasma Whip and Paralyzer more spammable, and more likely to create openings which did not present themselves when both moves were abysmally easy to PS, and the increased shield stun made both of them a lot safer on block. Safety on block has also been boosted for Dsmash, which was an excellent pressure move before, and is even better now. ZSS also kept her good spacing moves, naturally, all of which present openings; bair, nair, dtilt, ftilt. They can all present opportunities in the right spots.
ZSS does have some weaknesses, but mostly they serve to balance her strengths. Due to increased shield stun, ZSS's OOS options like utilt and jab are much less useful, and her aerials can really only be used to escape from a shield pressure situation. Of course, her grab is horrendous as always. This is another of ZSS's issues. Her grab is really only useful out of stun, and although her dthrow makes grab out of dsmash a good idea, it limits ZSS's grab game. A less laggy pivot grab would be nice, but it's unnecessary, given ZSS's other strengths.
Granted, I haven't played ZSS in 4.0, so some of my observations might no longer be valid.
