He has an outdated kit with some unhealthy interactions. Just because someone has AP ratios doesn't mean you have to build AP (I built none playing him and still did just fine.) Note he was very strong around the start of season 4 (And he still has a ~50% overall win rate) My thoughts on the mana changes is because he needs some weakness somewhere since his kit is actually pretty good.
Passive - Gemcraft - No notable issues. Has clear risks and rewards.
Q - Imbue - Other than the heals himself for more instead of healing his ally as well, not much issues.
W - Shatter - Armor Aura is a lot of hidden power and generally unappreciated (similar to Sona's old kit Auras) His armor shred doesn't offer much interesting gameplay either, it's also a bit of hidden power too.
E - Dazzle - It's a targeted Stun. Other than the distance it travels, no counterplay early game outside of losing a summoner spell (I don't think you should be required to run a spell specifically to counter one champion, given that you would be able to skillfully dodge or avoid effects.*)
R - Radiance - Similar case to Shatter. A good amount of hidden power that goes largely unnoticed and unappreciated. It doesn't have much interesting choices either; even if you aren't close to the other team to use your ult as a damage tool, you can just ult where you stand and give a stat boost.
As for itemization, I disagree completely. Spellthief wouldn't be easy to proc it's gold effect since he lacks reliable ranged harass, and the AP gained from it is only a miniscule benefit compared to 75 hp and the sustain/gold share proc it gives. If you want AP on Taric, you'll want something like Rod of Ages, Rylai's Crystal Scepter, or even Abyssal Scepter.
*And no I don't like Volibear's passive or Mundo's ult for requiring Grievous wounds/ignite to kill them.