As a sysadmin who works with Sharepoint Developers, I'd suggest basic understanding of the things you want to interface with, or rely on is what is important; I'm not suggesting you should be an expert on AD (for example) you should know enough about it to understand what it does for SP and how it is used so you can get the best out of it instead of viewing it as a magic box. Same for DNS, I don't expect developers to know the ins and outs of what different records mean but knowledge that DNS is important for deploying a new sharepoint site is good.
I'd suggest AD, DNS, IIS, SQL Server at least. If I was going to say spend a lot of time on just one or two of those I'd probably suggest IIS (its the building block for sharepoint after all) and SQL Server (poor appreciation of how SQL Server works and how Sharepoint uses it can cripple a Sharepoint farm as I'm sure you know).
I'd also suggest that a really good sysadmin might reciprocate the interest and try and understand what you're doing to some degree. As a sysadmin myself I know that the more the other people in IT understand what I'm trying to do (even if they're not clear on the how and the why) and the more I understand what those others are trying to do, the better the service we both provide to the customers.