I leveled my Draenei Priest all Holy. As Arp said, you can't make a comparison between leveling this way and leveling shadow. It's just not as effective, but it is possible.
If you want to go holy (there's nothing wrong with disc, btw) I would also recommend your first five points go into Spirit Tap in the Shadow tree. You can take them out at the higher levels, but to begin with, it will help you kill more mobs before needing a drink.
For leveling, I would also take the 5 points in wand spec from the Arcane tree. Again, take them out later, but they served me well while leveling.
As far as the holy tree itself, max out healing focus and imp renew, then move on to the talents that increase your holy damage and reduce cast time. If you're soloing alot, I wouldn't worry about the healing spells too much until after you're satisfied with your damage. You'll be able to heal yourself just fine without improving your healing too much. You will however, need to kill things quickly... your shied will only take so much damage!
Just my two cents.... whatever you do, enjoy it and have fun!
/rate up for Arp for not saying... "Level as shadow and THEN play how you want!"