At the moment, when enabling activities for an entity we get either all activity types or no activity at all. What I would like to suggest is the ability to limit what sort of activities are available to each entity.
For example, if I create a custom activity entity named 'Account Audit' which makes sense only for Accounts (no sense for quotes or orders, etc) then I wouldn't like the option to create this 'Account Audit' activity available only for accounts.
As a half-way sort of workaround, I do tend to limit the 'regarding' field of the custom activity form to show only the entities I want through the use of a jScript. However this isn't enough as users could still create the custom activity pointing to an incorrect entity from other contexts that would bypass the jScript. For example, I could go to the activities grid of a quote record and choose to create a new activity of type 'Account Audit'.