A rogue, definitely. The first two areas of the WLD are a bit static and repetitive, but it gets better later as the story aspect of the dungeon picks up.
I've used several sections of it as separate adventures. I set the first two sections in an isolated pair of mountain valleys, and made the goblins and kobolds each control one valley, with the dungeon sort of in-between and around them. The valleys were not big enough to support the two tribes, and they were in a death-struggle for resources. It was fun! The PCs were trying to cross the mountains and stumbled into the situation.
There's also a section (maybe G?) that is an ooze-walled region. I creeped my players out by letting them stumble into it. They had a way out, but were tracking down an enemy and did not want to leave. Then they got caught up trying to figure out how to kill the nearly unkillable monster (manticore?) in there.
So if you do allow them out, it is still a viable resource for dungeon adventures.