First and formost reason to avoid moving to Oracle: The learning curve. Mind you, some departments in Ottawa have banned Oracle because after they get a developer all trained up, we head off into private sector for 2-3 times the wage we get as a CS01. One course and one test to go for my OCP(just got otld, course is next week).
Other reasons to avoid Oracle:
Cost. I failed to notice anything in the budget that indicates that buying very expensive stuff will be covered.
Flakey behavior. This may be due to our development work stations being Win95, ver C, which according to Oracle, has not been tested and is not certified against. That gave us an excuse to try and get NT workStation, and as you know, it is end of fiscal, so there might be mad money available.

It is only fair at this time to say that I have never seen a more robust, stable and downright fast RDBMS. Or for that matter a more complex one. bandwidth seems to be the only thing that holds it back