Basically...
1. If you want to level fast, level as a DPS spec. Enhancement is clearly the better DPS spec at first. At level ~39 elemental is an option but still not better than Enh. At level 60+ after you grab all the early Outlands +dmg gear, I felt Elemental was better at questing/leveling than my buddies who went/stayed Enhancement, but both specs are viable.
2. As offspec you can still mainheal anything up til BC instances. Accumulate a second set of healing gear while leveling/instancing.
3. Google
"Joana's leveling guide" for extremely detailed descriptions of which zones/quests to take for optimal XP generation. Many other leveling guides exist, and I have no clue if Joana's is the best. It's merely the one that sprung to mind since someone mentioned it recently.
4. Nearly every instance has upgrades for you. Wowhead.com is my preferred site for using filters to search gear. "2h Weapons" (or "Mail") combined with "Found in Any Instance" combined with a "required level" spread near your own (24-30) will provide excellent DPS upgrade options. Maximize Str, AP, and Crit to maximize your leveling speed (generally in that order).
Edited, Mar 13th 2008 5:04pm by Axehilt