Leech: If you get hit, then you heal like 15-30 hp. Later in the game that is less than 5% of your HP pool... The talent is certainly helpful at the beginning of the game, but is far from being overpowered, it is actually a completely useless skill later in the game.
Leech currently heals for about 1/2 to 1/3 the damage I take from physical attacks. That is insanely powerful, even ignoring the other ways that Leech is broken.
I have a charcter with leech, it only heals about 15 to maybe 30 hp per physical hit. Not very much, basically worthless. Might be a good combo with glass cannon though.
Imagine if there was a talent that increased your HP by 200%. That would be ridiculously good, right? More useful than any other talent. Well Leech does that, AND MORE.
Except leech doesn't artificially raise your HP by 200%, not even close. And a lot of the damage that you take isn't even physical damage.