Thanks both. I could only play for 2-4hrs per week (most likely 2). That's not enough for an mmo even if I don't mind a slow pace, is it? (I've only played up to lv30 in FFXI and it was a nightmare)
If you're not concerned with getting to current content as quickly as possible and can only play for a few hours a week, you could spend a solid amount of time working your way though Vanilla, The Burning Crusade, Wrath of the Lich King, Cataclysm, Mists of Pandaria and Warlords of Dreanor PVE. You would probably decide fairly quickly whether or not it's something you want to do and could even take advantage of the first 20 levels being free-to-play to help you figure that out.
Wrath of the Lich King is where modern WoW starts to begin and the game starts to get more overt with its story-telling. The 1-60 Vanilla zones are sort of a hodge-podge, there's no over-arching narrative pushing you through the Vanilla's PVE, which was re-worked in Cata, but each zone has a self-contained story for you to experience. The Burning Crusade is some of the oldest questing content in the game and people tend to have a strong opinion about it one way or the other, but it does have some of the most inventive looking zones in the game, in my opinion. I fucking love Zangarmarsh. Wrath is where the game starts to transition from show instead of tell when it comes to the narrative. I don't think too much of Cata myself but what I've played of MoP seems absolutely brilliant, I wish I had started playing WoW back in MoP. Warlords PVE is pretty solid too overall.
Forsaken have a really strong narrative experience for their first 40 or so levels, Pandas, Goblins and Worgens have pretty strong starting experiences before you get thrust into the older questing content. You'd probably want to play as something that can easily solo content, so a Hunter, Warlock or any class with a tanking spec like a Warrior, Monk, Paladin or Druid.