1 and 2 were covered pretty well already but I'd definitely reiterate what folks have already mentioned about 3. The game is designed around playing your job after 30 for group content. Your job specific abilities are going to usually be your most powerful abilities and many of them are specifically designed to aid group play. If you start going into higher level dungeons as a basic class you are likely going to get some grief in the least and possibly kicked. I'd also suggest checking out a guide in the job you're looking to pursue to see which cross class skills are generally considered must haves and work towards them fairly early as well.