I swear I saw a similar thread not even a few days ago. I'll just repost my reply then here.
Taric's strength is he's good with and against AD-heavy teams. If they're AD melees, you're on easy street. He's also an aura god, bringing both attack and defense to teamfights that, when done right, can cause a massive stat disparity equal to a baron buff. His weakness comes from a general lack of peels and mobility and the fact that his armor shredding and buffs better benefit his allies than they do himself, though this is generally par for the course for supports.
Passive: Actually, his damage bonus is magic damage, but the really valuable component is the 2 second cooldown reduction. With a proper rhythm, Taric can lay down his spells constantly, refreshing all of his skills with such frequency that he's absolutely lethal to brawlers and melee assassins.
Q: Yeah, his heal could be better, but combined with his passive, he can throw down heals remarkably fast. Like, almost Soraka fast.
W: Shatter is one of the best skills in the game, but on a tank support it's almost broken. AOE magic damage and armor shred that scales off his armor is insane. I've carried a team as Taric support when the enemy is mostly AD assassin because, at some point, they can't do anything to Taric, and every second they stand near him is devastating agony, and he can trigger it so fast that armor shred is constantly refreshed. And the damage. DEAR GOD THE DAMAGE.
E: I don't like this skill, but unfortunately it's the closest thing he has to a gap closer, and if the enemy has any sense, they'll keep Taric as far away from them as possible, especially as he starts snowballing armor and CDR. It's just a flat stun and negligible damage. It's generally a negligible skill outside of the fact that it stuns.
R: The buff is amazing. It's like having part of a baron buff in the early game, and then a full baron buff in the late game. Two, if your armor shred is high enough.
Arguably, Taric is overpowered at the moment, but I think it's because of the extremely heavy emphasis on the AD assassin meta. He's the ultimate counter to it, and now that tanks have been given a general buff, it's penalized teams for being so specialized. Otherwise, against an AP ranged team (or one with high mobility), Taric falls off super quickly because of his lack of mobility or ranged abilities.