If you play many hours a day like me, you'd reach 60 in maybe 1-3 months, 70 in 2 or 3 weeks, 80 in the same amount of time, and 85 in about a week. It's a lengthy game, but you enjoy it. The Lore get's interesting in the later expansions, as in 70-85.
Also, nobody mentioned this yet:
In PvE, there is out-of-battlefield PvP, but you need to be flagged for this. You know when an NPC is flagged when they have the faction's symbol on the corner of their picture. You can tell when a player is flagged when their name glows a color. Flagged players of your faction have green names, Flagged players of the enemy faction has red names. You can flag yourself manually by right clicking your picture and clicking the Enable PvP option. If you are forced into PvP by attacking a flagged unit, or getting attacked by one, you will be flagged until you get out of PvP combat then you will be unflagged after 5 minutes of non-PvP combat. So basically yes, there are PvP NPCs.
Also, there is PvP armor and PvE Armor. The difference is that PvP armor has resilliance, PvE armor nevers has this stat, as it really isnt helpful in PvE.
Don't forget achievments, if you're like me, you love these. Most are just for bragging rights, but some can get you cool things, mainly tabards and mounts. With the release of Catacalysm, there are guild achievments, which are large achievments you need the guild's help to complete, these unlock BoA (bind on account) gear, which is used for easier leveling Alts (second, third, etc. characters). BoAs are basically armor that level's up with your character, as you level up, the armor does to, and it's stat bonuses increase, they are the best armor for leveling characters, they can only be given to characters on your account. BoAs are never dropped, they can only be bought via Guild Vendors and Trinket Vendors.