I've not reached level 50 with any class so can only comment on a few worlds. So I will
Coruscant - too long and too much running around. The Senate Plaza area, people, WHY is it so large? Aside from the merchants and cantina, there's doodly squat to see yet I have to waste five minutes just to get from one end to the other.
Don't get me started on the Works - just too much backtracking. Aside from that, all the areas look too similiar to one another.
Nar Shaddaa is similiar in feel but shorter, thank the Force.
Balmorra and Taris, both of these worlds have far too much running to and fro just to get around. I could really have used a speeder at level 20 instead of 25. Edit - The colicoids on Balmorra, I coudn't walk more than 3 paces in any direction without having to fight off a swarm of them. Annoying.
Dromund Kaas - at least you have speeders connecting the various parts of the city. I can take a speeder straight to whichever part of the Citadel my class needs to visit. I like that.
Tatooine and Hoth. I like the look and feel of both planets but there's still too much empty space to fly through. I suppose that's the feel BioWare wanted - they're both meant to be wastelands.
Quesh - Short and to the point. After Balmorra for the Republic side, it was like a vacation.