Your E attack is bonus magic damage. Lethality just cripples your damage by giving you a useless stat and giving a lot less damage than something more cost efficient. I also don't see why people go crit because your E attack, which is 2/3 of the damage, doesn't scale with crit. It scales with AD, so the obvious choice is buy an item that gives AD, not crit. Other than that, thresh is not a toplaner. You are so much more effective botlane where all you need to do is auto attack once to win lane for the adc. Of course, auto attack with caution. CC will destroy you and any form of burst that is higher than yours will also destroy you. Let your E charge up in team fights for max damage and disrupt the team fight with E, R and Q while you wait.