A great champion and my favorite and fun to play. His Q is amazing for chasing down enemies and letting teammates pick up the kill, his W is excellent (and you get a stack from landing his Q which makes things easier) and his passive is downright amazing (even with the fairly high cooldown) and he scales to health extraordinarily well making him a natural tank with a good execute. He is a good overall pick providing crowd control, (a throw into your team and a slow) and having survibility with his passive (and all the health that his W and passive scale to). However, there are a few things that i think Volibear lacks that he should have.
Although his ultimate procs off of basic attacks (which is supplemented by stacks on his W) its sometimes hard to pick up a kill using just that ability unless in a committed team fight. Yes, sometimes you can pick up the kill if they are faster then you by attacking a minion if your Q is on cooldown, but unless the enemy champion believes they can take you out, they normally flee meaning you have to follow up with other skills to effectively use his ultimate. (and lets face it, if you see a giant polar bear running at you suddenly spark up with 300 gigawatts of electricity, you're going to be scared ****less). The fact is, that if your opponent has an escape or decent movement speed, you're going to be spending time you could be hitting them chasing them down making his ultimate less effective.
(slows are excessively useful. a half second buff or less cooldown at lower levels to his E would be amazing, allowing more early game kills before picking up a frozen mallet or Rylai's Crystal Scepter)
although his Q is good in a team fight, Volibear suffers greatly against ranged champions and the speed boost is often not enough to close the gap before taking significant damage.
Having an armor rating of 16.5(+3.5), Volibear falls just above Taric (with 16.5 (+3.2). unlike other tanks he gets no additional protection from a passive or ability such as Taric's Shatter's passive, Leona's Eclipse's armor and magic resist bonus, Galio's Bulwark's armor and magic resistance bonus, Rammus's Defensive Ball Curl's armor and magic resistance bonus, Amumu's Tantrum's base damage reduction, Shen's Feint's shield, Alistar's Unbreakable Will's 70% damage reduction, Nautilus' Titan's Wrath's shield, Maokai's Sap Magic's 7% heal, or Thresh's bonus to armor from his passive. this means that I've named every champion classified as "tank" except Sejuani, who has 20.5 (+3.5) armor.
Volibear's passive is excellent for this, except if its expected. an ignite and a burst stop any chance of Volibear's passive from being as effective as it should be. (and then he gets in lane with his passive on cooldown)
(by increasing his armor rating to around the same as Sejuani's would be beneficial, however, since she is a ranged character, i believe Volibear should get a bonus to armor in a passive on one of his skills (possibly one similar to Taric's shatter on his Q) as well as an additional 0.5-3.0 increase to base armor as he is a close range champion allowing him to close the gap before getting a sunfire cape or locket of the iron solari)
Volibear is a good champ, but other tanks do it better.
Ultimate can be walked away from. slows are useful, his E could be used more often.
He is a melee champion with low armor and has to close the gap, ranged tears him appart.
Other tanks get higher armor, therefore, are harder to kill.
Volibear can be burst down from his passive, rendering it ineffective
good champ, but lowering cooldown on his E at early levels and increasing his base armor would help. he is a champ you have to build for, his early game is nice, but has flaws. even so, he is my favorite champion to play and he is just fun.