Not asking for Rito to change it but perhaps some clarity as a person who plays a lot of 
, why does 
and 
apply grevious wounds instantly on hit rather than like 
at only 35% hp?
the same reason 
needs to hit 9 targets with spells to get a spellvamp heal on her next spell and 
has it permanently?
I believe the intention is for burst mages to get you low, and at that threshold prevent you from suddenly healing all of their damage. It also prevents poke from being oppressive, since mages typically have a bit more range than autoattacking champions.
The AD items probably have lesser limits since sustained DPS should be able to out-damage sustain, making it harder for that Mundo to withstand a prolonged assault from an ADC.
Another note is that many mage items come with HP, while ADC (let's... not talk about AD assassins, please. I got nothing for them) champions have lifesteal as their primary defensive stat (although they might throw in a sterak's or a maw). Having a small HP pool makes it easier to get to that 35% hp threshold, and once you get there it'll make it harder for them to lifesteal it all back. On the flipside, they get GW easier because mages have higher health pools but typically less options for getting it all back (imagine having a huge health pool AND being able to get it back quickly while not being a tank!) quickly.
Just my $0.02
