As has been mentioned, Feral will be the most efficient tree for solo play (though you should spec how you want). You'll find good builds in the sticky. Key talents for feral soloists:
Feral Instinct - you may initially consider passing on this because you don't care about your threat, but it's huge. The stealthier the better for solo play.
Brutal Impact - for pulling off a heal in combat, especially if you're a Night Elf (and so don't have that stompie-thingie whose name is currently escaping me)
Feral Swiftness
Feral Faerie Fire - for pulling
Feral Charge - for catching fleeing mobs and interrupting casters
If you're putting points in Resto, Nature's Focus until you get Barkskin (after that you toss it in favor of Naturalist) - for uninterrupted self-healing, which is really key for solo play
This is of course in addition to all those lovely talents that increase your damage, and Mangle is a must for Feral druids no matter what they're doing. I don't necessarily recommend speccing straight to it (some of those Resto talents are mighty useful while leveling), but respec at 50 to get it.
For gear, you will want to stack STA, AGI, and STR (in that order), but as you're playing solo you cannot neglect your mana pool either. I played solo up to 70 and generally found that mixing in one piece of caster gear was enough.