I spent a good amount of time on the PTR this weekend, and from what I am getting from testing and from reading various forums, for the upcoming 2.4 warlock Life tap change is:
lifetap will now take 16% of max health and return 16% of max mana instead of 1:1+spell damage. Basically the bigger health difference vs mana the worse the LT returned. So any form of stamina buff is a bad thing now for warlocks. Since warlocks have no way of regening mana outside of LT and have the lowest OOC MP5 this is a big hit.
the warlocks (who could exist outside of a Shadow Priest group), now will require one to get close to where we were in DPS prior to this. We are going from a 22% of DPS time lifetapping to 30% DPS time Lifetapping.
Result - Lower ISB time - Lower Shad Priest damage - Lower Mana Regen for Mages, Paladins, Priests.
A few guilds have already theorycrafted the means of mitigating the loss of warlock DPS... Take a Shadow Priest AWAY from the Mages, Paladins and Priests and now give it to the Warlocks. The result is higher Raid DPS that without the SP/Lock group... Yet still a lower RaidDPS compared to live.
but this will be a mjor effect in the core of the warlock class and raid viablity. I personally care less about PVP anymore but in organizing groups locks will be low on the dps unless this is accounted for. And people will have to recheck their gemming and gear. Hello mage gear you are starting to look better all the time.