I wish it were true that Server OS couldn't be used on Desktop boards and vice-versa. Would make my job a lot easier.
Generally, there isn't much difference between the two. The only big ones would be video and audio support. And don't confuse what works with what a company may support.
Overall, the one we can say generally won't fly would be Notebooks with Server OS. A couple of clients have had this thought cross their mind since Xeon support has been made available on gamer models. Fortunately, most of the other add-on software needed for notebooks refuses to install or work properly on server OSes.