1. Variety - Moonkin (Caster dps), Kitty (Melee dps), Bear (Tanking), Tree (Healing), Caster (Getting killed).
   
2. Travel forms - You don't realise how much you love flightform until you don't have it anymore and level 30 travel form is very handy to have when you're leveling.
3. High demand for druids - One thing that hasn't changed since before TBC is that guilds love having druids in their ranks.
4. Fun to play - It is by far the most fun class available in this game. Once you've played a druid other classes just feel restrictive. I hate not being able to stealth up and sneak into rooms so I can mark mobs when I tank on my warrior.
5. You're cuddle (especially if you play a Tauren)

People have basically said everything constructive and useful that can be said about rolling a Druid; so allow me to troll a little:

5 Reasons to Roll a Druid:

1) You get claws.

2) You get to be a bear.

3) You get to be a bear with huge claws.

4)You get to rip things to pieces as a bear with huge claws.

5) Apparently they make good healers when they are not being bears... (but I wouldn't know anything about that!)

Via WoW Europe Forums