1. I've been saying that this looks overpowered on paper since the PAX videos.
2. While overpowered, I'm not entirely sure that it's as bad as all that. As Jerich points out, this is still an early-game boss. Don't overreact and nerf it too hard.
3. I've got no problem with charge-up making this skill ridiculous for a short period. Charge-up is an inherently ridiculous mechanic. I'd expect no less.
4. If what people have been saying about melee being out-and-out worse than ranged is true, skills like this might be closer to balanced on the bigger-picture level than they appear when examining the skill out of context.
5.a. Spreading the heal over a longer duration is probably the best nerf to start with, but taking it too far completely destroys the skill . Ideally, the heal/sec on all of your leech abilities put together (including this skill) should roughly equal the "wear and tear" damage/sec coming off the generic, non-boss "garbage" mobs standing between you and the next quest objective. A longer-duration nerf puts a hard cap on heal/sec, which runs a risk of making the skill worthless if average damage/sec from the monsters ends up exceeding the heal/sec cap. You will always have to pop a potion anyway. (For an analogous situation, visit a Guild Wars forum and search for threads about "why regen is worthless." There's a lot of them.)
5.b. Nerfing the overall amount healed per crit is the next place to go for a nerf. This doesn't cap heal/sec, so you can nerf it as far down as you need to. Note that increasing duration and reducing heal/crit together is going to have a multiplicative effect on the size of the decrease in heal/sec. A little bit goes a long way, so be careful.
5.c. Think about decoupling this skill from max-hp. On the one hand, coupling it to max-hp makes it scale and stay useful throughout the game. On the other hand, it enables abuse through hp pumping like doom mentioned. I'm not sure which is the greater concern.