Are there any downsides to installing Lion Server on my laptop rather than Lion? I'm trying to decide whether to get Lion or Lion Server. I'm not too worried about the $20 premium or the extra disk space used, but I'd rather not make things more complex to work with and manage, nor do I want my laptop to run slower as a result of having the Server version.

I'm not sure if I will use any of the Server specific apps in the long run, but I'm certainly curious to try them out. I just want to make sure that I can disable those services/features I don't use and that in other respects Lion Server really is just Lion + extra features.
Actually you have to have Lion to get Lion server which means that the cost is really $30 + $50. Server configures your computer to host wikis and mail server. It really isn't meant for day to day consumer uses. I wouldn't recommend it. 

If you want to try Lion server I recommend installing it in a Virtual Machine (VMWare, Parallels, VirtualBox) and playing with it that way.