Any reason not to allow automatic updating for an app? Since Android 2.2 (Froyo), you can select "Allow automatic updating" for applications. For some reason, I haven't started using this yet. The benefit would be to save time, but are there any good reasons not to use this feature?
I leave it off so that I can review the list of pending updates and see what changed.  Of course, that only helps for the apps that list changes in the market description.

Every once in a while, developers do release updates that are a step backwards- either because of introducing bugs or major redesigns that lose features (Astrid, I'm looking at you).  If you wait long enough before updating, then you may be alerted to that situation via comments.