We'll explain how this change works with a simplified example. Before, if you had 10,000 max health and 5,000 max mana, and Life Tap took away 2,600 health from you, you could expect to get at least 2,600 mana back. Now, with those same figures, assuming a 26% Lifetap, you'd still lose 2,600 health, but you'd only gain 1,300 mana. That's definitely a large decrease in mana gain.
With the Warlock's health pool now completely divorced from how much mana Life Tap provides them, the Warlock tradition of stacking stamina in order to be able to gain tons of mana at a moment's notice doesn't look as good as it used to for PvE. The Demonic Embrace talent, once one of the cornerstones of more than a few Warlock builds, now starts to look like something of a liability, as that extra stamina will now mean that you get hurt more from Life Tapping without gaining any more advantage in mana gain. In addition, with less mana per Life Tap for your average stamina-stacking PvE Warlock, Warlocks will be Life Tapping more often, which means less time spent casting actual damage spells, and therefore, less DPS. There's also a question as to whether or not Blizzard will properly revamp itemization for warlocks, lessening the stamina on their tier sets and drops in favor of more intellect for the larger mana pools they'll need to keep now. Of course, even as Warlocks retool their gear, and their health bar drops and their mana pool increases, there will be danger. Life Tap will become efficient again, but also a lot more dangerous, as they'll have less health to lose, and more chance of stray damage killing them outright.
It is likely that this change was meant primarily as a PvP nerf. One of the strengths of Warlocks is that even when faced with mana burns or long fights, they aren't completely out of luck, as they can sacrifice a bit of health (the main PvP stat anyway) for a bit more mana to keep going and keep fighting. With this change, the amount of mana gained by the Life Tap probably won't be worth the large amount of health lost by Warlock, especially since PvP Warlocks have massive health pools. It definitely seems, with things like the drinking regen change, that Blizzard is trying to make mana a much more precious commodity in Arena PvP, so the change would make some sense in that context.
However, in the end, it IS a very serious shift to the way Warlocks play, and will likely force most Warlocks to completely shift their gear strategies in the future. By shrinking their health pool and growing their mana pool and casting Life Tap more often, a PvE Warlock might be able to get back their old mana regeneration rate. However, it would seem to be a high price to pay, both in gold (to re-gem and re-enchant their gear) and damage done to them (more, especially in comparison to their new, smaller health total) and to the mob (less, because they're spending valuable DPS time casting Life Tap).
At the least, this looks like a change for the PvP game that's affecting the PvE game in a major way, and we'll see if Blizzard keeps it when 2.4 goes live, or offers anything more to balance it out or take a bit of the edge off for the PvE Warlock.