*CURRENTLY WORK IN PROGRESS. Trying to make this perfect from experience*

Thresh Can have amazing alpha damage if you go for Muramana/ attack speed / iceborne gauntlet.
Muramana means that you can easily hit 3oo+ basic attack damage.

The basic idea is to keep farming minions and to support your lanepartner (if you have one), while making sure you get Manamune to transform into Murumana.

Once this happens your potential damage output increaces greatly when you toggle on Murumana. Remember, the bonus damage is proprotional to your MAXIMUM mana, so invest in more mana, this will also increace your basic damage without Muramana being turned on. This means that you can give relativly high damage while still conserving mana, thereby keeping ready for your alpha strikes.

Armor, Magic resistance, attackspeed and gold.

Pros:
High alpha damage mid to late game.
Good at grabing and taking out single enemys.
Works well in team fights, especially with the use of the lantern and the ult while still dealing large chunks of damage.
Easy last hits.
Good farm all though the game due to the long range and high basic damage of his attacks.

Cons:
Slow to get going.
Scales in large increments based on items.
NEEDS Muramana, which takes time to transform.

Both his high damage and his long range make insure that Thresh is good at getting last hits.

Once the Iceborne Guantlets are done, groups of enemys die even faster.