The old Improved Sap took 3 points and got you a 90% chance not to break stealth when you sapped a mob...the new one is 2 points and 100% chance not to break stealth. The talent is also now much lower in the tree...you can get it after 5 points invested in Subtlety.
So what's the big deal? Well a common annoyance for rogues who pickpocket mobs of equal level for either coin/gems/potions or, more importantly, lock boxes to skill up on, is the random chance that your pickpocket is "resisted"...dropping your stealth and ending you up in a fight that's not always fun (multiple even level elites for example.)
The solution is to just sap the mob first...there is now NO chance that you will break your stealth and the sapped mob can't randomly move around (causing you to miss the pickpocket or accident "see" you), and more importantly if your pickpocket fails because of a resist...nothing happens! You are still in stealth because pickpocket resists don't break sap!
I've never noticed anyone posting this, but I will apologize now if this is common knowledge and practice among rogues...mine is still just 55 and this is a trick that I just organically developed, but I'm certainly willing to believe that others could have as well.