Having a strong foundation is paramount. There are plenty of areas to specialize in the SQL Server realm, but the job(s) you have had usually dictate what you continue to learn and become better versed in. I also try to keep up with the general ideas of as much as I can, so if an opportunity arises to employ one of these "new" (new to me) ideas, I have a good starting point. I have landed some jobs on my ability to learn new things quickly and impliment them, kind of jack of all trades and master of none, and I have also landed jobs on the expertise I have aquired on the job in a particular area.

Bottom line, it is all in how you market yourself, whether to a potential employer or to your current employer.