How similar is D3 to D2? Want to know before I purchase

Hi there ladies and gentleman, I love D1 and D2 and its expansion, I've played D2 at least 3 different times and beaten the game, what I loved was the gothic and dark style, the character progression, the simple, but super fun hack and slash fighting, etc...

I didn't get D3 early on because it was light and rainbows and no character progression, too much enemies and effects on screen, seemed like too fast paced imbalanced game.

My question is has the game improved with all of the patches and new content and stuff and how different is the Vanilla Diablo 3 compared to its expansion, meaning would I have a different experience playing D3 vs its expansion Reaper of Souls?

I've heard that the expansion really makes the game good, so I'm afraid that D3 is still crap and you need to buy it together with the expansion for it to be good.