So you know when you're feeling really guilty because you picked Alistar and now you're feeding over and over and you feel pretty much useless? Well, its ok its not your fault Alistar is in a horrible spot right now. He has the second lowest win rate in the game at the time of this writing at 44.35% per lolking.net. Alistar is one of those champions Riot has a hard time balancing. His kit is designed in such a way that it will never be balanced by mere number changes. Alistar has fluctuated from OP to worthless with just small changes (just like Sivir, Olaf, and Mundo). When he is strong enough to be playable he is way too strong. This happens because Alistar just has too many spells that have no counter. Pulzerize and Headbutt both are not effected by Tenacity, and Alistar's ultimate makes him immune to CC and extremely tanky. His ultimate is similar to the old Olaf ultimate in that, enemies have no way to stop you. Against most champions, a player can CC them or kill them and they can't attack them anymore, but that's not the case against a spell like Alistar's ultimate. To 'fix' Alistar when he was too strong, they nerfed his ultimate's damage reduction a LOT, and increased his mana cost on his E (the heal) over 3 different patches until now he's at the point where he has serious mana issues. Alistar needs some love. They fixed Olaf's ultimate by making him more squishy in exchange for him being immune to CC. I think the opposite would work for Alistar, if they removed his immune to CC portion and just made his ult make him even more tanky he would be in a good spot. That is really what you want anyways. Then you can lower his cooldowns, reduce his mana costs, and improve his base stats. I think Riot fears doing that now, because as I said he would suddenly be OP again because his kit is just so strong in it's current state.
I think a lot of people misunderstand me when I make rework proposals. They always look like nerfs because I am removing things from a champion (see my posts on Sivir and Rammus). And well they are nerfs at first, but the reason champions are usually underplayed or weak is because they have some really imbalanced portion of their kit preventing them from fitting into the game. Once that is removed, you can buff numbers everywhere else and make the champion playable. Riot knows this and they did it with Sivir. They removed the attack speed bonus to teammates on her ult which was OP in certain situations, and reduced the damage on her Q which was insane. In return for that she got some base stats, an improved W, and much needed mana cost reductions. I still disagree with a lot of changes they made to Sivir (her ult giving movespeed to teammates is still too strong IMO, and I miss her Q damage), but I understand what they did and why, and now she's in a pretty good spot, being played competitively and in solo queue. Many people seem to think Riot is just sitting around watching players complain about their favorite champions needing buffs and laughing at them. Balance is complicated though, and if Alistar hasn't received a buff in an entire year (last patch from Jan 16, 2013), it's probably for a good reason.
If you read the forums and Rioter's logic on buffs and nerfs in patch notes, you will see there are trends appearing. Certain champion traits make champions very difficult to balance. Best examples: Tenacity/CC Reduction (Dr. Mundo, Irelia, Alistar, Olaf, Poppy) and Globals (Shen, Twisted Fate, Janna, Zilean). All the previous champions are either super weak or super strong and many have fluctuated between those points for years. Riot knows these problems exist, but removing these types of abilities entirely takes away some interesting parts of the game, so it is a challenging spot for them. I think they are on the right track though with the Olaf rework. Riot gave him a trade off for his OP ability to ignore CC by making him more easily killable by straight damage. My point is: even though many champions seem like they are in a horrible spot and are unplayable, they actually need nerfs to their kit and not buffs to be playable again. After you figure out how to nerf a champion's OP ability without destroying who they are, you can buff everything else.
3/10 Edit: Going back to this I think I overestimated the value of the tenacity on his ultimate. It's not even really tenacity as it only breaks CC on casting and does not make him immune to CC as I stated above. I think Alistar's real issue is the power from his Headbutt and Pulverize. They can't be reduced by tenacity, and Headbutt provides so much positioning power. Also Headbutt is very difficult to use for lower level players (i.e. not LCS pros) so that causes him to have a lower winrate in solo queue, but if they just straight buff him, he would be too strong competitively. I think he needs some kind of change to his Headbutt and Pulverize to make them easier to use without trying to do the W+Q combo, which is needlessly difficult.