So Kayle a great champ if decently fed, okay if played normally, **** if not. Horrible early game, decent mid, great mid-late, good late. Sounds good right, wrong she is horrendous early game, and if she is not fed she will have troubles for a while, her ult is disgustingly short with a long cooldown, her heal has one of the worst healing ratios in the game, she has bad base mana regen, and horrendous base AS for a melee attack based champ, also she really only deals damage late game when she uses righteous fury.
I think that she needs only 2 changes to make her good: AS boost it should be around 1 at lvl 18, and make righteous fury a toggle with a high cost (so she can have sustained power in fights, but can't rape continuously). These changes would make her stronger and also give more last hitting power.
She won't get a healing boost cuz it has a speed boost and Riot is not showing love to invincibility.