Advantage/disadvantage is great.
It's got just the right amount of character customization to me. I don't like when character creation takes too long or is too complicated, but I don't like the cookie-cutter characters of truely old-school D&D, either. The amout of customization 5e gives you between your race, class, subclass, and (optionally) feats is juuussst right.
Huge amounts of 3rd-party support, and simply free fan-content on the Internet.
"Upcasting" (expending higher level spell slots to boost low-level spells) was a fun idea.
Legendary actions, legendary resistance, and lair actions mean you can finally actually have proper boss fights in D&D. Took them 40 years to figure that out.
That halfling from upthread aside, the artwork is gorgeous. It's classic fantasy, which in a weird way is kind of refreshing nowadays.
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Crap, you know what? I've talked myself into it. I'm between campaigns at the moment. I guess my next one will be 5e.