Bluesky asks about which server to use on registration like mastodon.social does. For me the difference seems to be better marketing for Bluesky which makes people want to deal with the change and somewhat enjoy it rather than see every obstacle as a pain.
Bluesky does have some advantages like starter packs (and network effect now that it has so many accounts).
Mastodon typed in almost any search engine points to mastodon.social; in some cases joinmastodon.org shows up on top which still suggests mastodon.social but has a distinguish "pick another server" button on right side.
Bluesky by default creates account on bsky.social, while there's barely noticeable button with icon that allows you to provide another server.
An ordinary user will head to what it'll find in the search results, fill out credentials and log in. It won't change the defaults if it actually notice there's a possibility to do that at all. Not mention a mass user needs to be aware in the first place that there are other instances/servers, and you can interoperate with these just like with email providers. Neither mastodon or bluesky explains that in a simple discoverable fashion - which IMO should be done long ago.
one cool difference between Bluesky and Mastodon (et al) is that server choice on registration is not an immutable permanent decision and you can choose to seamlessly migrate at a later time by updating your DID document
so the slick registration flow nets you less lock-in compared to if e.g. the mainstream Mastodon app were funnelling users onto one megainstance, since you can still get away afterwards without needing people to re-follow you
You're right, when I tried again the field was hidden behind a link. Last time I checked (sometime last month) it was a field that defaulted to bsky.social (maybe within the account name?). Or I could be misremembering.
Yeah - technically the same button, but with a slightly more subtle treatment. People were getting confused by it and bouncing. IMO, since it's only really useful if you already know what you're doing and already have a PDS ready to go, there's no harm making the first-time experience smoother.
This is all totally going to change when OAuth rolls out, so hopefully we can find a better compromise then.
Bluesky does have some advantages like starter packs (and network effect now that it has so many accounts).