I plan on doing 90% dungeons once I hit 19 or so (horde side) to 60. But this is only when the servers are fresh (1-4 months). I assume that the servers will be jam packed with people farming single mobs so it may be a smoother time just to do dungeons if you are not leading everyone in levels.

The only time I plan on doing quests after 19 and outside of dungeons is if its 'right there' and easily farmable...for example, quests that are geared for groups will more than likely be less populated.

Gathering to me is a 'ill get it when I get it' kind of rule when grinding. Even though you can get 300 herb/mining while leveling, I find it very beneficial to just level as it takes too much effort until you get a mount. Sure, its great to gather things while you level IF its nearby, but I certainly wont go out of my way to collect something when the XP is flying.