Hi Rob,
A sys-admin resource center with or without a forum would be great.
You can use your experience and knowledge to put together a solid website.
And you could certainly realize a nice price in the future with a great domain name, developed site, numerous backlinks, solid PR, etc, if you decided to sell it.
And no question that "root user" is a popular term on Google. This is the result that I received when I did a Google search:
Results 1 - 10 of about 6,050,000 for root user.
I would not be in a hurry to sell the name, my friend, unless you really need the cash. The name will just get bigger in time.
Good luck.
Take care,
Kevin