We have implemented this as a custom solution but I think this would help CRM adoption greatly.

Allow the use of the Activity Dialog to be automatically be presented when someone clicks "Mark Complete". The user can elect to not create another activity, but if they do select a new activity, the information from the current activity is copied to the new activity.

We have further customized this by adding two Option Set fields to the Activity. They are called "Topic" and "Topic Completed". "Topic" identifies the Topic the Activity is about. "Topic Completed" identifies if they were successful or not. In our system, if "Topic Completed" is Yes, then we do not display the Activity Selector Dialog. If the "Topic Completed" is no, we do display the "Activity Selector Dialog". Lastly we implemented a startardized, read-only subject that uses the Topic and other key pieces of information to build a Subject that looks consistent. This allows us to review the Activities and in a quicker, more efficient manner to determine the actions that people have done Activities. The subject line is not editable unless the Topic is set to "Other". This ensure consistency but allows for unforseen events.

If you implemented the whole thing I would suggest allowing developers to modify the fields copied between Activities so they could customize it. I would also suggest allowing control of which Activities are available in the Activity Selector Dialog. This allows us to track how many Activities it took a person to accomplish a Topic. It also reduces the amount of work it takes to make new Activities when you were unsuccessful at your task.
As we have already done this in a custom solution, I don't think it would take very much work on Microsoft's part to implement this. I think it would greatly increase efficiency for the CRM users.