im just past level 24, and im a good ways thru the "main plot" vegas missions (one of them has you connect with factions and do shit for them so that quest is more like a gate to a bunch of other quests)
and really there is a SHITLOAD of stuff i havent touched yet.. so im thinking once I hit 30 (which is the cap) im gonna pull the trigger on the endgame scenario so i can still have a bunch of stuff to do in the second playthrough
btw - the main plot can go alot of different ways, and i mean ALOT... so you definitely wont see all it has to offer in one play