I don't enjoy the prospect of altering humans in a significant way - the general ethical and more pragmatic worries ("what unknown effects could snail genes have on us?") stuff doesn't really factor into my own concerns, more the societal consequences.
I'm in favour of modifying humans if it will save them from a life-threatening disease, or cure some sort of disability. I don't think anyone would refuse a blind man the opportunity to see again. However, my problem is that everything with humans is a slippery slope. We love pushing boundaries, we do it from childhood to our last years. This can be positive: black rights > women's rights > gay rights. However, in this particular situation I see it as negative. Curing blindness turns to curing subpar vision turns to boosting a dwarf's height to turning "John who's always felt a bit short at 5 foot 3" to a 6 foot man. Treatments that start off as important turn into cosmetic. Soon, everyone is modified in some way, because they felt like it.
Now perhaps that's not a problem inherently, but I think it would cause something of a rift in society, arguably on a huge scale. I'm not a conservative person, but I'm against this. If that's the case, do we really think conservatives + FAIRLY liberal but weary people aren't going to be against a "post-human" movement? Considering the likely rapid evolving of such a thing, it could (I stress *could*, this is perhaps a bit extreme) get to the point where the "conservatives" and the non-conservatives form a different class. Regular humans can't compete with altered ones for jobs and so on - they'd be like very disabled people today. There'd also be pressure on people to change their kids to give them the best chance in life etc, I just don't think it'd be healthy for society.
As I said, that's perhaps a tad... apocalyptic. However I do think it's a real concern and on a smaller scale, an inevitability. So I need to ask: do we need to do something with such massive consequences? I'm all in favour of needed progress that has some negative repercussions, but I don't know if post-humanism is anything other than progress for the sake of progress.
If there was some sort of way to ensure that throwing animal DNA or w/e into people would be done only for necessary stuff like immunity to lethal diseases, curing crippling disabilities and so on, I'd be fine with it. However, I just don't know if it'd be able to be kept to that and I do not see the need for anything beyond the two examples of healthy usage I listed above.