I would say that the game is super good for cauals now. It is stupidly easy to level and now you can gear up super fast, with dungeons easier and loot far quicker to obtain.
Basically original wow is all you need, and will take you from 1-60. The Burning Crusade will get you to level 70 (and add the outlands, which is an entire new mini world if you like, as well as give you the option to be Blood Elves if horde (and so paladins on horde side) and Dranei on alliance (and therefore shaman class on alliance side).
After that is Wrath of the Lich King - which will allow you to level to 80, and raid the "end game" content. (as well as the large north rend area)
Once you hit mid 70's you can do most 5 man dungeons for gear, you will then get to 80's, and do heroic 5 mans (which give you epic gear, and badges to buy even better gear than that) and probably naxx 10 man (super easy now - gives tons of gear) then ulduar, and trial of the crusade.
Overall, you can get to 80 pretty easy. If you ask for class, a paladin or Druid are great options because they can do everything (druids can even fly without a mount, can stealth like a rogue, can DPS long range and short, heal and tank). But be what you like.
Need info on something more specfic, feel free to ask
