Imprinting takes the idea of "love at first sight" to a completely different level. Of course, the primary criticism of imprinting comes from the three major imprinting storylines:
1. Sam & Emily: Sam imprints on Emily, Emily rejects him, Sam grievously injures/disfigures her, she accepts him. The implication here being that the abuse theme begun with Bella/Edward is being carried to a conclusion: women should love their abusers so they stop abusing.
2. Quil & Claire: Quil is physically mature, Claire is two. If imprinting is primarily sexual, how can this not be an endorsement of pedophilia?
3. Jacob & Renesmee: Jacob is physically mature, Renesmee is a newborn. See the above criticism.
Again, I believe most of these criticisms stem from people not taking into account the alternate reality in which this story is told, but I'll admit that the imprinting aspect has really been the only thing about the series that has given me pause, upon reflection. (When I read them the first time, it didn't bother me at all.)
The Sam/Emily imprinting didn't bother me, and still doesn't. I frankly don't see what the fuss is there. The werewolves unpredictability is documented thoroughly throughout the story, it is entirely possible that Sam could genuinely lose control and injure Emily. It is also possible that bonding would take place between them during her healing process.
I don't believe the relationships between Jacob/Quil and Renesmee/Claire are abusive, but these make me more uncomfortable. I don't believe that imprinting is only sexual, though naturally it has some sexual component. What bothers me is the idea that all a woman really needs from a relationship is complete adoration from a man. I don't have my books to hand, but Jacob, explaining it all to Bella, says something like, "Why wouldn't she want him, in the end?" and "That kind of adoration is hard to resist." My personal experience has been that love has to be much more mutual, so the idea that a completely one-sided infatuation can blossom into a meaningful and lasting relationship is hard for me to swallow.
Of course, we never get any perspective from the female half of the equation. Renesmee seems devoted to Jacob from the beginning: is there a "magic" at work on the female half as well? And did Emily just resist it out of loyalty to her cousin, Leah? If the imprinting impulse worked both ways, it would explain a lot.
And put my mind at ease.