When I played WoW I hated anything that made me go in a cave - quests, travel, whatever. Dark, cramped, and usually only one way in/out.
I think that's why I really disliked Coruscant and, even more so, Nar Shaddaa. I felt the same annoyance after a few levels on each planet as I did questing in caves in WoW. My first character was a Trooper so the first four planets got more and more infuriating. What irritated me most about Nar Shaddaa was that it felt exactly like Coruscant, but with some green ambiance and Huts. It felt lazy. Maybe, as a previous poster said, if they had varied the first few planets it wouldn't had bothered me so much.
Nar Shaddaa was bearable as a Sith Warrior. I think it was because of having DK and Balmorra early to get my fill of open-worldyness, it felt shorter as a Warrior, and I generally enjoyed playing the Warrior more.
TL;DR
Nar Shaddaa, especially on the Republic side.