Where to start?

As far as I am concerned starting a new Character must be based on your personal style of play.

For example:

If you like standing back and attacking, DO NOT ROLL a warrior!
If you want to get right in on the action, DO NOT ROLL a warlock!

I have tried virtually every class now and have enjoyed the various challenges they all present.

Paladin: Great all rounder and although many state that this is not an end game class...I would rather have a plate wearing healer in many HIGH level instances than a clothie!
Druid: Depends on the spec ofc, but the Kitty I have puts out BIG DPS!
Mage: Ranged spell based DPS with CC capabilities... fun turning mobs into sheep tbh.
Hunter: Imo... easiest class to level if you use the right talents and keep up with the acquiring of different "pet"skills along the way
Shaman: *yawn* mine never got over level 26.
Warlock: Tough but a great challange. Also simply unstoppable if played properly and well specced and geared
Priest: You will have to ask the King about this class... not one I have bothered with.
Warrior: Again, have a chat to Tankabull... another class that does not interest me (although I will probably roll one at some point)
Rogue: Oops... another one I have not played. Corpsey or Tanakbull can help with these till I roll one.

I will be posting detailed information on every class and add notes on those that I have rolled and what works for me!

Watch this space

I'll try and answer as best I can.......

Priest:

As shadow spec its unbelievably easy to lvl up, with crippling amounts of damage output in the form of DoTs (which also heal and restore mana to you and your party/raid) also not bad as CC against humanoids (mind control a enemy during a raid on Halaa and run the sucker off a cliff for massive laughs!)
CAUTION! Avoid melee like the plague, your phsycic scream will only protect you so much and your shield spell will only take a couple of belts at best.

Holy spec does exactly what it says on the tin! Holy priests are the best healers in the game...no doubt! but if a melee opponent even so much as sneezes in your direction? you are dead! Having said this ....the Holy priest is actually capable of huge amounts of dps if done correctly and because of that, they are actually not bad to play solo and lvl up. Oh....and if you are holy spec....expect to get an invite to EVERY dungeon there is, 10 times a day!

Rogue:

I have limited experience with rogues so you would probably want to get specific advice else where but here's what I do know.
Rogues are the kings and queens of PvP, every other class hates them! They put out massive dps and can keep another player in almost continual stun while they stab the crap out of you! With my pally at full strength and no spell cool downs I can hold my own agains an experienced rogue, however, if my spells are on CD or the player is good enough to get that first stun in...then I'm boned! and I have over 13,000 kills with my pally so I know a trick or two
Rogues are also very handy in PvE too. They are capable of long periods of CC in the form of SAP and being able to sneak around and find those pesky pats ect is invaluable. Oh....and that awesome dps is a heck of a bonus too