Different things depending on what we are hiring for. It also depends on how people present themselves. Algorithms, protocols, software design, data structures.
Hell, there are times I'd be happy if people just understood the difference between TCP and UDP, or knew how to make an HTTP request. I guess it would be a lot easier if we were just looking for code monkeys to follow orders.
I would guess very basic stuff, dns, http, webserver ( not anyone in particular, just what they do really ), smtp ( apparently it's magic ), object orientation, software design in general. You'd be surprised how many people a doing web-development without knowing how the internet works.
) Being able to tell and smtp server from a POP3 or IMAP server is a rare ability.
I've noticed that about Android Developers. I prefer iOS development but have a little more professional Android experience and at least 3 times a week I get emails from recruiters about new positions. I think it might be time to really focus on Android.