Mostly new features I guess? And not just making the base game better. For sure improving the base game and old features is a mandatory thing, but that should not replace new features, which it does in Dragonflight. I can already see how divisive Dragonflight will be just by how basic and ordinary it is for a new expansion.
Tier sets returned in Shadowlands already, so I'd hardly call that anything specific for Dragonflight. New content aka zones, dungeons etc. is for every expansion a given and nothing unsuspected. It's the bread that carries every expansion, so is improving the base game. But where's the butter? As I said, Evoker is it and if you like that - like me - you're lucky. If not, Dragonflight is like a piece of dry bread. It will satisfy your hunger, but it won't make you feel anything besides that.