Lol. Yeah, if you go glass cannon duskblade/rfc oneshot build, you need to go for picks, or stand way back, land a hook halfway thru the fight, and assassinate the enemy carry with one auto. I like to sometimes do essence instead of dusk first for the 30% cdr on it once I get rfc, then play a little more passive till I get dusk and start oneshotting.
With blue build its sorta similar, you act as peel for adc while doing a fair amount of damage, try to hook the adc or enemy assassin/mage, or anyone actually, the hook is good in general, but here its more imperative that you get the enemy squishies in the ult box so that theyre slowed to be killed. the ult does a lot of damage too.
you can go for a shred build with bork cleaver and wits (ive done it jg too for bloodrazor) then get guinsoos if youre fed or go for tanky items, then hook whoever. Youll be doing a lot of hybrid damage, with all souls collected adding 1 magic damage on hit, + all items you might have. youll be able to kill their frontline as quick as their backline, at a decent sorta pace. Frozen mallet is real good with this
with fulltank thresh top you might as well have gone supp, but you enable another high utility champ to go supp, and still offer a lot to the team with the utility, which is why I play thresh in general. Youll be a huge damage soak
Hes very versatile. Even if you don't want to be in a fight, just throw a hook at the fight. if it lands whoever you hit is out for like 2 seconds, then you can decide to jump to it, like a lee sin q but with cc. you can also e while you have someone hooked, or hook as you cast e. he provides a lot of lockdown to the team, even with full damage. the lantern utility is always good, and helps with jg clear and saves lives more than youd expect with the shield, and if youre really good you can use its tiny collision box to make enemies stutter step to catch up with them and there are places you can place your ult if youre fighting in jg to completely zone away enemy squishies/ assassins entering from odd angles. also your souls ensure infinite scaling, and by lategame can get you the equivalent of more than two armor items if you stack them, not even mentioning the ap and on hit damage.
The coolest thing I ever did with thresh tho was on accident. I had always known thresh e could stop dashes, but one time I played thresh into riven. Thresh's e has an instant cast time on the side that pulls towards you. So when riven would go to knock me up, when I saw her in the air I could e real quick to pull her towards me and it would end the animation, cancel the knockup and the damage.
I think it works well cuz of all the things you can bring to the team. Hes a self reliant champ too. He can play any role, the only one I haven't done is mid.
Also, I play karma top. She's op af in a 1v1 situation
and lose the lane automatically unless they massively fuck up or your jungler camps them because they can simply keep clicking on you from range and safely proc thunderlords or fervor, and to a lesser extent DFT and Grasp.
over 
is if you use a mana-less champion
as well.