Quite often, I am repeating the same set of joins in queries (both normal and advanced), that I would ordinarily put in a view. Think e.g. of a CRM system that regularly needs "all active clients" or an ordering system that needs "all open orders". Of course if you need just that, it's easy to put it in an action, but I often find myself needing the result and join that with some other tables.
Unfortunately, for some reason OS does not allow to define database views, which imho is a great gap. Therefore, I'd really like to see support for database views or something similar (e.g. queries you can define on eSpace level and join the result of with another query, but that amounts to the same).