Feral is the most common build for Druids while leveling up. Most talents in the feral tree boost both bear and cat forms, but a few are specific to one or the other. Solo, you can fight longer in melee forms and save more mana for healing. Since you don't have to stop and rest you will rake in the experience very quickly. With a friend or two you can spend more time in bear or cat form depending on the strengths and weaknesses of the other class. In instance groups you can be the main tank and use your skills to keep the attention of enemies away from your squishy friends.
Focus in this tree should be new abilities, damage increases, and reduced costs for your favorite skills.
Key talents:
Feral Charge - immobilization and spell interruption, absolutely nasty in PvP
Faerie Fire (feral) - a cost-free ranged ability for pulling and making Rogues hate you
Leader of the Pack - 5% crit boost to everyone in your group
Mangle - an extra attack that is devastating in any situation
Step by step:
Level One - Ferocity is amazingly powerful for a level one talent. Feral aggression is decent for raiding Druids in later levels, but is not considered as valuable as Ferocity.
Level Two - Tough choices already. Druids who love cat form or pvp usually take Feral Instinct and Brutal Impact (one second seems like forever in pvp). Tanking Druids might prefer some combination of Thick Hide and Feral Instinct.
Level Three - Feral Charge is used more often in pvp than pve. It should be a given for any build though. Sharpened Claws is strong on it's own and should be maxed out here because it leads to Primal Fury at the next level. Feral Swiftness is great for leveling Druids who don't have a mount or Travel Form yet. Pvpers also get great use out of the extra maneuverability.
Level Four - Predatory Strikes leads to Heart of the Wild at the next level and so that's a given. Primal Fury is also a given. In cat form it is a huge boost to dps and helps bear form tanking. Shredding Attacks helps in groups and pvp so you might want to come back for it in later levels.
Level Five - Savage Fury is a huge boost to Mangle, so only pass it up if you aren't going all the way down. Faerie Fire (feral) should be a given unless you spend all your time raiding with other druids who can keep FF active. Nurturing Instinct is questionable. Even a feral druid shouldn't be playing main healer in strength gear. This is a good level to pass up skills for some that you liked but skipped at earlier stages.
Level Six - Heart of the Wild is arguably the most valuable talent in the tree. Get it. Survival of the Fittest is very strong for tanking Druids. It will allow you to build agility and stamina in you tank gear instead of defense.
Level Seven - Leader of the Pack is such a large boost that you should get it even if you spend all your time running solo. Improved Leader of the Pack is also highly valuable in solo and raiding, though pvpers often take Primal Tenacity instead.
Level Eight - Predatory Instinct is a modest boost to cat dps and maybe worthwhile for a raiding main tank. Many Druids go back for talents they passed up at earlier levels or throw in just enough points to get to Mangle.
Level Nine - Mangle will become your favorite ability in cat and bear form. Very few feral druids decide to go without it.