Leech, unfortunately, suffers some pretty heavy nerfs. First off, there's a global leech penalty (-50% in NM, -66.7% in Hell). After that, each monster has its own little "Leech Resistances". Skeletons, for instance, are immune to leech. Other monsters have leech reduced a further 50% or more. In other words, even if you have 100% life leech in Hell, often you'll only be leeching 10% of your damage, if that.
It's worth noting that Lifetap suffers no nerfs- it leeches at full effectiveness regardless of difficulty or monster leech penalties.