Anyway, two recent Eyonix posts have discussed the topic. First, he clarified that Improved Life Tap will work the same as in patch 2.3: you get more mana without losing more life. Zero talent points in ILT = 15% of your max mana for 15% of your max life, one point = 16.5% mana for 15% life, and two points = 18% for 15%.
Second, he confirmed the suspicion that like so many other annoying changes, this one was made for the cause of PvP balance. He states specifically that "we wanted warlocks to take a larger hit to their hit points when lifetapping in PvP." Just like the water issue, they're again claiming that this change will have little impact in PvE. He says this could even be played for a gain by stacking Int instead of Stamina -- but doesn't most good Lock gear have a lot of Stam? (Honest question, I don't know much about the state of endgame Warlock itemization.)
Really, can't they just make this ability work differently in PvP than in PvE, like they ended up doing with water? It might be confusing for the players at first, but I'm sick of Blizzard breaking my PvE game to balance the PvP game, and I'm sure PvPers occasionally feel the same in reverse. It's gotten to the point where if I could press a button to remove PvP from WoW forever, I would consider doing it, so they would stop screwing up the part of the game that I enjoy. But it doesn't have to come to that! Just balance the two parts of the game separately where necessary.