I do think that G+ is going to have some very tricky issues to navigate ahead in terms of integrating with other services. Private messaging is a decently amusing one (I wouldn't be surprised if some thin layer on top of email is introduced for that), but what about something like Facebook Events? They're a very compelling, very sticky, bit of engineering. Google Calendar has the functionality required, but it's not presented in the right way for this use case. but do you want to go that far? How much of GCal gets pulled in to the G+ interface? How much of the GCal underpinnings do you expose? All/Some/None?

Google has all the engineering bits and bobs in place for pretty much every Facebook feature, but it's unclear whether the vision is of G+ as a bit of glue between services, or whether it's the veneer on top of all the services. It's also not clear which would be best for users in the long-run.

It seems that, for G+, this is just the beginning, and I do not envy the G+ team :)

DISCLAIMER: I'm a Google Intern, but have no dealings with any teams here, nor do I know anything about future features.