Making an application "managed" is a big step. Immediately several items become frozen and fixed for all time. Although many of these make sense to permit upgrades, many are unreasonably restricting and do not allow for "normal" evolution of an application over several versions.

Take one example: the text comments in a help popop on a custom object field. They can't be altered after going managed. If user feedback says that the comments aren't helpful enough - tough luck. If the use of a field changes - again tough.

This example is one where the AppExchange system could permit the developer to make the change in the package - and then all future installations of that package have the new text. Anybody that has already installed that app and customised the popup help text would keep that customisation. The change to the managed package would only affect new installs.

There are several examples of the above intelligent approach to managed package upgrades.

Another big example is to permit items to be depreciated. This includes custom objects, fields and scontrol. These would be flagged as "no longer used". Or better still, not included in new installs of the package. Like the previous example older installations would not have the depreciated item automatically removed (although they could be given to option too if they wish).