Some classes are substantially better, others have been cleaned up. For example, the fighter has an entirely new set of tools, from Second Wind now being +1d10 to any skill check, to indomitable being a pseudo legendary resistance; Barbarians gain new bonuses from their rage, including advantage on stealth; rogues get to flavour their sneak attack with extra effects at the cost of their damage output; Warlocks have bigger expanded spell lists which are automatically added to your known spells, and a few extra invocations; and so on. Additionally, most of the optional features, invocations and maneuvers from Tasha have been included, too.

There have been a few nerfs as well: +10/-5 feats being removed/reworked will drastically shake up the meta for martial classes. Unfortunately, we have no word on the main problem the game has faced since its inception - spells, which apparently are not going to be playtested. I wonder what the designers have in mind; the same designers who tried to give infinite Wish to three different spellcasting classes as a capstone, and an ungodly broken metamatic to Wizards. Yeah, not holding my breath for that.

They also decided to leave the rest system unchanged, while also putting some band-aids on certain short rest-reliant classes (Warlock and Monk) instead of fixing the rest system as a whole. So if you play a Warlock, you can recover half your spell points in 10 minutes, but if you're a fighter stuck with a DM that doesn't use short rests on a regular basis, you are still going to suck.

It's basically just a Tasha's cauldron of more things, but the options presented are no longer "optional" and are baked into the base rules, which is good because having your class/subclass fixed in an optional ruleset that your table may not accept was not great.