The main storyline should (by Yoshii) take a single class from 1 to 50. That doesn't include the various side-quests, guild-quests and grand-company-quests. Questing will be an excellent way to level in ARR compared to FFXIV1.23 where PL (low levels), grinding parties (40+ parties that could include lvl.50s) and leve quests were the best ways. Also, you all seem to forget the lvl capped F.A.T.E events (including chocobo runs with an unlimited number of players now). I believe this will also be a good way to grind and gather whatever rewards it will give.

Questing will definitely be a good alternative to level in ARR (main story = 1 lvl.50 job is damn good) but there will be a lot of other ways... leves, FATE events, grinding, PL-ing, grinding, dungeons... Which one will be faster? Nobody knows but there will be plenty of different things to do to level which gives you more options to choose from than you had in FFXIV1.23.

More alternative ways to level...THAT is the main thing to remember here.