I honestly think it helps to get every class to around 30 just to get the cross-class skills, but that requires patience and a fair amount of grinding. The cross-class skills come in handy for all your classes, but you don't really feel all the options until your class hits lvl 20 and you have a good number of slots for them. So, i'd probably hold off leveling alternate classes until 20 (15 if you really want a specific skill).

Also, Managing aggro is a little different in this game compared to WoW. It is closer to Everquest where a player has to spend a few moves to build it up before the dps and healers can safely do their thing. The warrior does get a lot better later, but you will definitely want their aoe aggro build skill, flash (it is an aggro build skill, and should not be treated as a taunt), for later dungeons. Thankfully you do pick it up as a requirement to become a warrior. I would recommend reading the tank forums as they go quite in to a bit of detail about the actual tanking mechanics.