I blogged a while back about an article I wrote for our website intended to help administrators who are not new to messaging systems but are new to Exchange get on their feet with the basic tasks of Exchange administration. We got a comment from the website berating us for not having anything about security in the list, and although it didn't fit 100% with the goal of the article, it was still a good comment, so I added a task with information about securing the server.
That made the top ten list go to 11... I tried to get the web team to put a Spinal Tap reference, but they have to think about things like localization so they weren't so willing to play along (shucks). So we ended up removing one of the tasks and putting it on the intro page instead.
Anyway, it's been updated, and changed into a step-by-step list to make it easier to read: http://www.microsoft.com/exchange/techinfo/administration/TopTenTasks_1.asp