I've levelled Three
I leveled FERAL and raided RESTO on an Alliance Druid in Vanilla (back when Innetvate was out 31 point talent) Hard days, but I liked my Druid then. went from Molten Core to late AQ40. Anything I would say about this experience would be obsolete, so I wont.
In BC I switched to Horde, I levelled FERAL with a rogue friend and absolutely carved it up (talent changes make feral much smoother the second time). In BC I raided RESTO from Mag (skipped Gruul) to Sunwell. I switched back to FERAL and tanked heroics and raids to KT/Vashj in late BC for a change.
In Wrath, I levelled my Horde druid 70-80 as BALANCE with my brutal arena gear. Failkin levelling was quite fun then. Though less fun than with the new 4.0.1 talents. In early Wrath (Naxx>>ToC) I raided RESTO, cleared naxx, Uldar to Yogg, Toc full clear. I switched to FERAL for ICC and tanked every boss in the instance. I have also ran every boss in the instance as RESTO. FERAL tanking was amusing with Swipe off cooldown and Maul hitting 2 targets for full damage. FERAL tanking is still fun ffor single tanking, though currently, multi-mob tanking is a bit of a chore. At 81 we get thrash, a second AOE that will solve much of the current AoE issue. RESTO healing presently is a bit boring. The talents and intellect changes gave me 15k more mana, but my spell costs hardly changed. Right now, healing is a joke, come Cata, it will be balanced properly, and I look forward to this.
Currently I am levelling my 3rd druid (lvl70 now) with BoA cloth, weapon, trinketsx2. I am levelling it BALANCE outside of instances and RESTO in instances. Outside instances, I demolish anything, balance changes + eclipse mechanic + powerful dots + an understanding of Eclipse whoring mean I can keep myself in a Nature state for almost 100% of my BALANCE levelling (which means buffed moonfire (with talents that make it nature), buffed insect swarm). Inside instances, I BALANCE healed with 0 issues/zero wipes until I could buy dual spec, then specced RESTO. Swiftmend is your core talent and it is utterly OP from the get go. Rejuv ticks with the new talents are doing 20-30% of a tanks health per tick too. Efflorescence sould be eared around the time you do Mara and it makes instances very smooth (especially when you get to BC content and you start getting 2-4 DKs in each group). Mana is stupid right now too, With the BoAs, single pack pulling will never drop me below 95% mana. Double pack pulls are easy and triple pack pulls mean you might need to innervate or drink once or twice per instance.
If I had to pick a spec I liked the least, it would be BALANCE. I cant bring myself to respect it as a DoT caster or a DD caster. The mechanics are fun, but I will always feel this spec to be inferior, and the least useful of our specs. In this however I am biased by having a warlock, who, In my experience was top caster dps in Vanilla (There was only one Mage in my AQ40 progression guild who could *sometimes* surpass me). Top DPS in BC (I was top guild dps from Mag to Sunwell, the only other contestant was another warlock who was able to beat me after I started raiding as a druid). In Wrath, I was never serious as a warlock, but I was usually top dps through ToC and ICC (skipped Naxx and Uld due to late levelling)
The one spec I have never played is FERALCAT. While post 4.0.1 I have played around with glyph changes while CATing in BEARgear, I honestly cant comment on this spec.
TL : DR = You want a class that is varied, can level whatever, can change however they want, can fill whatever role is needed. Be a druid. We are delicious.