I personally loathe a good Thresh. The only reason I don't mind seeing him all that often is because there's a decent chance the enemy Thresh got stuck with support and picked Thresh because he's the "best."
My biggest bitch with Thresh is that he can single-handedly relocate you from your turret to the river in a 3-second cc chain (hook > flay) while his adc blows you up. At level 6, you end up in the Box slow.
The hook range plus the flay knockback plus the damage and cc duration are just way too strong for how short the CDs are. It's way too much reward for landing a single skillshot with an enormous range and large hitbox. By comparison, if Morgana lands her Q on you under your turret, her adc has to extend further, deal with your support, and potentially take a turret shot or two to secure a kill.
Thresh relocating you such a long distance means Thresh's adc can stay at a lane position relatively safe from ganks while separating you from your support and removing turret from the equation altogether.
I think Thresh's hook would be far more fair (given his overall kit, which is bloated as fuck) if it required him to actually drag you backward by running the other direction, similar to Skarner. This would accomplish a few things at once:
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Cut down on bloat by forcing him to prioritize movement speed to maximize hook effectively
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Let Thresh make picks more safely late game in exchange for no combo into flay
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Make him less "master of all trades" in lane by leaving the hook distance between him and his target when the chain breaks, leaving a few seconds before he can get back in to attack the target.
Having a full, strong kit is perfectly fine so long as there are meaningful tradeoffs involved. Right now Thresh doesn't have to make any choices between damage, cc, and relocating the enemy. If he hits a hook, he gets all of those things at once. If he misses, he tries again in a few seconds. He doesn't have to build AP to get AP. He doesn't have to build armor to get armor. He doesn't have to build MS to avoid getting caught out. He doesn't have to extend to go for a kill.
As a general rule, other supports don't enjoy the luxury of getting free damage stats and free defensive stats outside of cooldowns (Leona) or at the cost of heavy early game handicap (Bard). Overall, I think he gets too much at too little cost. The support game is about who can do the most with the least gold, so it makes sense that a champ who gets free AP, free armor, and free movement is going to be an obvious contender over champs who have to spend a few thousand gold for all of that.