gosh... most things you girls hate - I couldn't live without. I've only got a 1 story home, but I love our monitor. I even bought a 2nd one! Then when DS is asleep, I can hear him wake when we are in the living room watching loud TV. And like one other... I have slept through his crying because I am so sleep deprived!
Most things I've bought (mostly on eBay) have been terrrific...
I've put some thought into things that I don't like/use/want:
1. High chair that doesn't fold flat (bought one on eBay & sold it.)
2. Hats. I have so many baby hats - and my DS hates them.
3. ditto for mittens & booties.
4. blankets & wraps that are too small. DS hated being wrapped, but when I did wrap him, I needed big ones to really tuck him in. He just wriggled out of the little ones.
5. basinette linen. I used the basinette for 12 weeks which was a godsend.. but really didn't need much linen for it.

edit: just remembered the most useless thing I got - three of them actually!!!
BREAST PUMPS. I bought one before I gave birth (so I had it ready). Then we realised in the hospital that I wasn't producing milk, so he went onto formula. But I hired an electric pump from the hospital ($10 a week) - (can also be hired from lactation clinics). Then I really tried to produce milk, so I bought two more pumps - as each of the manual pumps were uncomfortable. I barely got any use out of any of them. I would hire one first. And if you really needed to pump then either buy an electric one or a really good manual one.