This passive is awesome, But can also be a weakness if you don't get enough farm, or you're up against an AP heavy team (as he doesnt get MR). The collection of souls adds magic damage to his Auto attacks (makign him harder to itemize against), gives him free armour (good in general, especially good against most Top champs), and gives him FREE ability power. This can make up for the lack of AP in items, and with around 100+ souls, you get 80-90 AP, and your ulti still does 650 magic damage for the first wall, and half that for the others.
The souls can sometimes be hard to collect if they are too far into your lane, however, if you're playing him right, your opponent should be zoned and you shouldn't have any problems. I you absolutely cannot get to the souls, your W can be utilized as a kind of "soul vacuum".
Q: Death Sentence
This is what makes AD thresh viable in the first place, and what gives him so much friggin poke. So, the PASSIVE for this ability adds magic damage to your basic attacks. HOWEVER, it isn't a set amount of magic damage. The longer thresh goes without attacking, the stronger his next Auto attack will be. the extra magic damage goes up in intervals, with 5 levels of damage (and a base amount of damage on his autoattacks, which is determined by the amount of souls he has collected). From my observation also, the first level (80% of AD), seems to be applied to regular auto attacks as well. So if i'm right, your regular auto attacks gain magic damage equal to souls collected + 80% attack damage (will do further testing and observation and update). That is a decent steroid!
poking is also super easy. essentially just strafe around till you have 200% stack (the square is red), then AA your opponent, and continue to strafe out of range. If they back off a fair bit, just farm minions. If they decide to come back in, continue to poke. they quickly are reduced to 1/5 health and are forced to sit under tower :)
the ACTIVE for this ability is so versatile. It has two functions. 1. You can activate it once, stun them for 1.5 seconds while pulling them towards you. 2. you can reactivate Q and get pulled to the enemy (think of Amumu's Bandage toss). This makes an awesome initiate, especially while chasing. It can ALSO be used as an escape. You're running away from enemies in the jungle, you see a mob camp up ahead behind a wall, just q through the wall and pull yourself to the minions, continue running. This is super effective, and most enemies dont expect it. You can also use this ability in lane, to either pull the opponent under your tower (if they're hanging around pushing and you'r kinda low), then drop ulti, push them into it with your E, and if they're still not dead, use your exhaust to keep them under tower till they die. I have not only got easy kills this way, but have managed to thwart tower divers while at 1/4 health, and get multiple kills by kiting and keeping them away with CC :D
It also works for Tower Diving OR Tower sniping (similar to Blitzcrank's Q). if they are low, you can throw your Q at them and either just 1 hit them with it, OR pull yourself to them quickly, attack them and get a kill, and get away. A really sneaky way of doing this, is sitting behind the terrain next to an enemy tower with an opponent at low health. they can't see you, and probs won't expect a scythe to appear through the wall and THEN A THRESH APPEARS THROUGH THE WALL AND THEY'RE DEADDDD. It works, and its fun. try it :P
I max this first.
W: Dark Passage
This is an AWESOME ability, and is pretty much unique. It DOES share some similarity with Lee Sin's shield, in a kind of a reverse fashion, and an AOE shield instead of Single target shield. Essentially, it's basic function is to provide a AoE Shield. This is good for when you are by yourself top lane (think riven's shield), or later on in teamfights to help with a bit extra mitigation. The SECOND function of this ability allows an allied Champ to right click the lanter, and then TELEPORT TO YOU!! This is such an awesome function. This can be used to teleport your jungler to you isntantly for super awesome ganks, you can throw it over a wall so a teammate can teleport to you and not die, can throw it on the ground, use your q, get pulled to your target, and your ally can come with you, allowing TWO OF YOU to jump on someone. This is such an awesome ability.
the WEAKNESS of this ability, is it's second function requires someone else to interact. if you're playing with randoms, or someone just isn't paying attention, your possible saves can go to waste. It's ok though, the shield by itself can save someone :)
Also remember, if you go outside of the radius, the lanter is immediately returned to you, and not only you get a shield, your teammates get one too. so you can potentially throw it behind you to scare/confuse your enemies, and receive the shield after running a moderate distance. this works too :D
I max this second.
This ability is a great aoe stun and movement CC ability, and includes a slow. Great for peeling, securing kills, or even just disrupting the enemies in a teamfight. Dont rely on it for damage though, it doesnt do that much. And remember that it's range is that large in one direction, but it's total length is actually quite large. I put one point in this third, and then max it last.
R: The Box
This ulti is so OP, and makes Thresh even scarier. It was an amazing AP ratio (1:1), allowing it to retain decent damage from the souls collected, while building AD. as with most of his kit, this ability has several uses.
this ability creates a 5 sided box around where thresh is standing. The first wall does 150/400/550 magic damage (plus 100% ap), and slows by 99%!! (OMG WTF SO OP!). The remaining 4 walls deal half damage, and slow by half. The same champ can't be damaged by multiple walls, however I believe the secondary slow still apllies (might be wrong, let me know if I am).
You can use this ulti to trap aggressive opponents under a tower, killing them easily ;), you can use it while running away to either save yourself or an opponent, you can drop this in the middle of a teamfight to deal nice aoe damage and slow everyone down. you can use this in a 1v1, when your low opponent starts to run, and they get super slowed and they are dead :)
its a great ulti. it has a relatively long cooldown, and a set mana cost. It is great throughout the game :P)
put point in this whenever possible (level 6, 11 and 17)