So, here's my two cents, for whatever it's worth...
Most of the people in my network of friends and family simply don't shop for gifts online. It's not that they never buy things online, but if you're going to a baby shower on Sunday, you're probably going to buy the gift on Saturday, not a week or more ahead of time. They like to walk into a brick and mortar store and buy a set item within a couple of days of the event.
My guess is that if you register just on Amazon, then most people will buy you some cute little baby item (likely clothes and blankets) that they can easily find at a local mainstream store.
So, I would recommend registering for some things at BRU and/or Target and expecting to buy items like cloth diapers and slings yourself. If you have close family members who will be spending larger sums of money, you may talk to them about buying some of those things (or simply giving you funds for these things). On my registry in the notes section I wrote something like "She will be a breastfed and cloth diapered baby" and surprisingly no one gave me disposable diapers, bottles, or pacifiers.
Here are some things that I personally find worth registering for/buying from BRU and/or Target:
Car seat (infant bucket and/or car seat)
Bouncy seat (excellent for keeping in the bathroom so you can put baby down while you pee or during your showers)
Pumping/bottle feeding supplies if you plan to WOH
Safety items - cabinet locks, top of stair gates, etc... these can really add up
Stroller(s)... This can be pretty overwhelming, and really depends on what your life style will be. Often you want more than one stroller for different things. Even if you primarily baby wear, there are times when a stroller is a wonderful thing to own so you aren't stuck wearing the baby and carrying all of the baby stuff.
A changing pad to go on top of a low dresser unit and covers to go with it (I personally love having a changing table set up and would recommend using a dresser top so it's furniture that you'll still use as baby grows)
Bath supplies - Target and BRU both have California Baby and Burt's Bees products. We never used a baby bathtub, but you might want one.
Nursing supplies - breast pads and lansinoh (for awhile after DD was born I gave this to pretty much every new mom. I went through nursing pads like crazy, and every new mom should have something like lansinoh on hand.)
Basic medicine type items - Infant Motrin, Hyland's Teething Tablets, a thermometer, an aspirator, baby nail clippers, etc
Car things: a rearview mirror (shatterproof, a bit soft, and big), a large sunshade for the rear window, etc
Crib linens and such: if you plan to buy a crib and/or set up a nursery, items like crib sheets, mobiles, etc - you could even register for a crib, a crib mattress, a nice glider, etc. BRU has the arms reach co-sleepers if you're interested in that.
I personally have found that people love to give cute baby clothes, blankets, towels, etc, but they like to pick them out themselves. Also, the stock of these items varies greatly from store to store as well as over a period of a month or two making it really hard to find the item you registered for. I personally wouldn't bother to register for much along those lines.
Both Target and BRU offer some sort of discount on items from your registry that you don't receive by a set date. So, it might be worth registering for things that you don't really expect people to spend the funds on. Then you might be pleasantly surprised, or else you'll get a discount. You're also rather likely to get a couple of gift cards to those stores anyway.
FWIW, assuming we're having any sort of shower at all, I'll go ahead and register for a few things that I would plan to buy anyway even though we're having girl #2.