with samuel we had diapers, a blankie, clothes, a sling, and then a ton of stuff I didn't really need like a bouncey seat (took it back b/c he hated it) and lots of other stuff I gave away.
I would say bare minimum:
diapers, wipes and covers (there are a lot of threads on diapering a newborn at the diapering forum right now. Just search for "newborn")
carseat (I liked the infant carrier style, though we didn't use it as a carrier much)
waterproof pads for your bed (the "crib pads" at the stores are hot and uncomfortable, so I'd try the ones that www.motherofeden.com
nursing pads (I want to try wool ones this time)
baby clothes (I didn't use all the onesies I bought either)
we did like our swing a LOT, I must confess, and although we didn't buy a crib, Samuel ditched the whole co-sleeping idea at 6 months old, so we ended up wishing we'd had one. I use the stroller a lot now that he's a toddler and I'm pregnant, but considered taking it to a resale shop when he was about 6 months old. We just NEVER used it!
With this baby, I'll have about the same stuff. I did register for some things, like a ton of pacifiers (your call, but after three weeks of him sucking on my pinky finger, I wanted him to take a pacifier!), a wipes warmer, some baby hangers, baby towels and other odds and ends, including a booster seat for Samuel.
I would say to start with the bare minimum, or if you do go all out, don't take everything out of the package and wash it all, making it completely unreturnable like I did.
You're so smart to be thinking about this kind of stuff now!! All I could think was "WOW! I'm finally pregnant; I want this and this and this." My natural mama plans evlolved from simply being "slightly AP" to being all out AP over my pregnancy and now most people would probably consider me "crunchy." However, this time I *am* decorating a nursery, even if it never gets used. With DS we didn't have the money and were renting out house, so I couldn't really do much. With this one, we own a three bedroom and that room has NURSERY written all over it!!