You can certainly level-up by just doing dungeons and roulette. In the time you're waiting for queues, you can do levemates or FATE events. It may seem like small amounts of exp on the greater scale, but it adds up quickly.

Admittedly, the DPS queue goes slow, but I'm doing Ninja now and it's not all that bad. If you're standing around waiting for FATE or Queue pops, you're doing it wrong. Move around to get the best benefit.

I've got too much on my plate, so I'm always busy doing something - but I at least use the Low Level and Guildhest Roulette once per day to maximize the time online for low level jobs. (I've maxed out Tank and Healer classes + Bard already... working on NIN > MNK > BLM > DRG in that order and they'll all be done before the expansion hits.)