What's wrong with Life Steal ? Why should we change it ?
Simply it gives too much. You either have life steal and most likely win or u haven't and probably gonna lose lane trades/teamfight. Not every champion can get LS because it's just doesn't suit his build ( Garen , Darius ). Or you don't want to buy LS, you want replace it with another great items that you like. But you have no choice.
How we should change it ?
Reduce life steal on creeps. Halved, for example. This will decrease sustain at all. So people will be free to build another great items over LS.
Make life steal to be affected only by your 100% AD. Combined with crits, LS gives too much survivability for carries. Besides crit chance is already depends on luck, the crit with LS grant's even more reward for being lucky. Furthermore champions like Vayne , Draven , Caitlyn benefits more from LS due to abilities that enhances their AA which gives them big advantage.