As a Diamond 5 thresh support main, I feel my opinion bears some merit. It is true that thresh has an extremely strong kit. I've tried to make a guide on mobafire for him, but wasn't able to, simply because the correct way to build thresh is to build how the team needs you! Thresh brings everything to the table a team could want: heavy crowd control, the ability to peel, high damage, and an escape for his ADC, as his own escape if you use his Q cleverly.
However, Thresh is not overpowered. The advantage of Thresh is his versatility; he can be used as a blindpick any day without question because he fits into most any team. Thresh has many disadvantages in response to this, which I list below:
* Thresh falls off after the early/midgame.
Thresh's main advantage is his E/Q combo at level 2, which can all-in most support/adc combinations of the day. However, if the enemy dodges the combo, a huge powerspike is lost to Thresh. The same goes at levels 3, 6, and 11. After these points, thresh's entire kit relies on landing a hook, which despite what someone with less experience might tell you, is easy to avoid.
* Thresh has clear counters and comparable supports to battle with.
If you think thresh is the only one who plays the way he does, or is the strongest in lane, you aren't playing right. Thresh has equal or lesser strength to braum and leona, and is flat out countered by morgana and alistar, who can negate his combo's. Thresh isn't leagues ahead of these other supports; I could take any one of these others and have comperable results.
* Thresh relies on his team's lead and his earlygame to win.
Thresh's main strength is his powerful laning phase and deadly earlygame ganks. After this point, however, he becomes useless if his team can't find a lead. In fact, if his team is behind, Thresh can dig a deeper hole for his team with misplaced hooks, stupid dives, attempted rescues, poor roams, and baits on his team. In addition, Thresh's passive, collecting souls, makes his armor fall off of the other supports' levels if he can't reach them.
* Thresh can't peel as well as other supports.
Thresh actually has poor peel. Don't believe me? Just try keeping their tank off of your ADC when you're behind, and we'll see whose right. If you compare the peel of Thresh to someone such as leona, braum, nami, janna, morgana, alistar, or even Karma, you can see how he isn't amazing. Any of these other supports has a better method of peeling, usually with the aid of a targeted ability. Leona? bash with shield and jump on them. Braum? Auto them so they can be stunned by your ADC. Nami? Heal your adc, make them move fast, and pool the attacker. Janna? Play as janna and get freelo. Morgana? spellshield your adc and root them for 500 years. Alistar? Pulverize and headbutt. Karma? Root, slow, move fast, and shield to peel them. By comparison, Thresh isn't as strong as you might think. All of his abilities are either skillshot or rely on him positioning away from the fight, which you generally don't do with thresh. If you miss your Q, you have a 10s CD. If you miss your E, you're screwed and have to use your ult. If you throw out your lantern, the enemy can just toss a ward on it and your ADC is screwed. To be fair, Thresh isn't hopeless for peeling, but the point is that most other supports are better, other than thresh using his lantern to help an out of position target.
I like to compare Thresh to Lee Sin, or irelia: usable in practically any composition due to their strong earlygame/midgame, but falls off during the lategame to a position not nearly as effective as other choices.