I noticed a few things that are sub-optimal when joining a new community.
When you are creating a new profile on a site with an existing network profile, the default behavior is to apply a display name, bio, location, URLs, etc to the profile that comes from the existing account. Although this may seem like a good default, there should be a way to opt out of it. Personally, I prefer using the same user name (which happens to be my real time) on the different communities, but I can definitely see a place where people may want to have better isolation between their profiles on different communities while still maintaining a single network account.
I'm not sure where this profile comes from, since it's synched at the Stack Exchange level across all sites (including stackexchange.com).
However, I'd propose a method that allows you to optionally import your profile. The default could be use what is in your Stack Exchange profile, but ideally you should be able to import it from any site on the network or import it from no sites on the network.
In the event that you do not import the profile from an existing site, the default behavior should also be to hide the community from your other profiles. It should be difficult for someone to link your newly created profile to your other profiles on SE sites.
I believe that this will help people who wish to remain more anonymous and use the network in different contexts. Perhaps someone who wants a certain display name (perhaps their real name and bio) on sites they use in a professional context while a pseudonym on sites that are related to recreational activities. It can help improve privacy across the network, and should be able to leverage much of the current profile synchronization tools that exist today.