OK I typed way much more than I had initially intended, guess I needed to get it out of my system, I don't expect anyone to read all this if at all, if you're interested you can skip most and just read the tl;dr, toplane or versatility part.
PASSIVE and STUTTER STEP: Old Taric's passive sure did less damage than the new one, but one hit is in many cases better than two; it allows for better stutter-stepping in between abilities, thus you are able to stick much better, even retreat while attacking. Even with double AS the single hit comes out faster initially, and allows better movement between passive procs, if you wanted more dps you could still buy attackspeed, something I loved to do - turned you into riven without mobility lategame due to the need to cancel spell animations.
This increases reliability and even versatility A LOT. Which sometimes leads to more damage since you can actually stick to your target. The only thing in Taric's kit that really does sustained damage is the passive which only has a 10% scaling increase over the old ability, most of the damage comes from levels.
VERSATILITY: As Sad Tyrant pointed out, Taric really lacks versatility, and he lacks reliability:
- His defence spell Bastion, only shields and provides some armor (very poor early game).
- His heal only heals for a miniscule amount and cant really be compared to healing from other champions and thats all it does, it also has a 45 sec cd, which is only reduced by proccing your passive, thus you constantly need to be in combat or you need to waste spells on minions. If you are ever in a position where your opponent has an advantage and zones you out somewhat, your sustain goes down heavily.
- His utilty spell, Dazzle, is your primary damage in trades, but it is also your only waveclear, and your only CC, and your only proper defence (bastion wont save you). It has so much it has nothing, if you waveclear with it you will get initiated on and lose badly, you need to choose between keeping it as defence or to try and initiate. The ability is so telegraphed and has such a long delay that its very easy to avoid, Taric is so immobile you cannot properly capitalize on the stun if it does land.
- His ult.... is really not good in a 1v1 scenario, at least not in laning. The delay is too long to be impactfull when you are at low health, if you ult half-way the enemy will just... kill you, or walk away, stun on cd since you need to use it at the start most of the time. Even if it lands in fight, 2.5 seconds is pretty short for how kitable Taric is.
- The passive seems great on paper, but as I've mentioned you cant stick, its too reliant on both parties standing still so you can just dps, that almost never happens.
*Cast times (not delay/channel). I might be wrong about this, but I've played old Taric a lot, and the new one seems to have much higher cast times on his heal and shield, this in itself makes the passive AS boost misleading. If a window of 2 attacks and one ability takes 1.5 seconds, it feels as if the ability cast took 0.5.
TOPLANE: Taric is a terrible toplaner, he lacks the neccessary tools to survive and trade there (note this is mostly in 1v1):
-Poor, situational sustain.
-Poor, avoidable defence.
-Poor waveclear.
-Poor combat pattern, no real choices, almost completely melee. Damage very avoidable and predictable.
-Poor mobility, low base, no boosts/dashes.
-Poor initiation, very telegraphed and delayed stun. Poor followup to said stun due to no synergetic abilities and low mobility.
-Very linear and unrewarding, if you hit it you might get a decent trade in, if you miss your stun you are done for.
-Scales off enemy ignorance, even harder than Garen/Xin.
-Increadibly punishing with little reward, bad trades leave you with no way to recover in lane. If you get in a good trade, a quick look at your mana bar makes you realize you can only trade once more. If your opponent doesn't die in 2-3 trades, you have nothing left and become vulnerable.
-Increadibly punishing again, even if you miss a stun, or waste a shield, and your enemy doesn't punish you, the mana cost alone can lose you your lane presence, and make it absolutely neccessary for the next stun to hit. Early this makes it impossible to even trade.
-Feast or Famine, strange for a support/bruiser type champ. You CANNOT play from behind, if you gave a kill or two, its over, you cannot come back, at least not in lane. In teamfights your team will be seriously lacking damage if behind.
At the same time, if you got a few kills, the enemy can't even come near minions. Your passive alone will destroy them. He's pretty much the definition of stat check, funny since people considered the old one to be just that.
+Decent DPS, burst is ok, but even though his damage can range from ok to really good, in a combat scenario you will never get it off, its completely melee (even 150 range which isnt great) you are immobile and it isnt helped by huge cast times that cut into your mobility even more.
+Absolutely ridiculous scaling into lategame, don't think I've ever lost a game where I got my Items and the game went late. You outdamage about any other bruiser single target, while giving your team some of the best utility.
RISKY STRATEGY: Problem is these things force Taric to play incredibly passive in lanephase, he cannot afford to lose a trade, he has no fallback and falls behind too easily. I guess you could take a Taric top in a 5 man as I did mostly where your team will build around you and your jungler can give you a lead, but it's a very risky strategy and in soloq it will be very dodgy.
Maybe you can change my mind, but I've spent almost a year trying to make him work top like old Taric, to little success, I think I've finally given up.
Old Taric was also weak early, but he countered a lot of melee AD's, he also had tools to easily freeze lane with shatter, had really good and reliable sustain, maybe less damage but the increased reliability and shatters range advantage over AA's made it possible to trade effectively, his stun could be used defensively since he didn't need it for the damage. His ult was a great steriod in 1v1 fights especially with the armor reduction from shatter, it was reliable, it was verstile; aoe burst, increases dps, increases sustain. Becomes a powerful anti-bruiser lategame, divers or frontliners can't simply ignore him to get to his carry, he will stun-lock them and kill them easily.
DIFFERENT CHAMPION: But you can't really compare the two, they're not the same champion at all anymore. Only partly the reactive playstyle and the passive remained the same, even his theme changed drastically. I despise the rework personally, for this is a new champion. It makes sense he'll be bad toplane, his entire kit is balanced around 5v5 teamfight, aoe heal, w value doubles, aoe dual cast stun, long range aoe invulnerability, passive to do it all again (minus the ult). I don't find pleasure in this playstyle but unfortunately there's no replacement for the Gem Knight.
Wtl;dr
Taric has a terrible time in the toplane.
He lacks reliability and versatility; his skills mostly offer only one thing, except the stun that has too much, making it too punishing to miss, considering how easy that is, it becomes a bad idea to use it at all unless reactively, forcing Taric to play passive whole lanephase. His passive does good damage... if the enemy stands still, which will not happen with experienced opponents. Taric cannot play form behind, and even though he has a hard time winning lane, if he does the enemy can't do much either. Hes a very polarizing champion, but with huge disadvantages early, making him a weak and unreliable pick.
Honestly the old and new Taric simply isn't the same champion. Almost everything, most spells, theme, playstyle, niche, all changed. The new one is almost completely balanced around 5v5, it makes sense he'll be terrible toplane.
when fed?
was torture, playing against a tank such as 
ranges from ok to even easy, and for some strange reason 
was a cakewalk.
