OpenACS gives you a huge head start in development time, even if the application you're building isn't like one of the existing modules: bboard (this discussion forum), new-ticket (bug tracking system), intranet, ecommerce, general-comments, polls, etc. (Of course if you CAN use these modules you realize even larger savings.)
If nothing else you will want to use the users table defined by OpenACS (and the user workspace pages in /pvt) and some basic user handling functions, e.g. ad_maybe_redirect_for_registration and ad_get_user_id. Writing code to handle cookies is boring and it's already been done for you, so why reinvent the wheel?
If you ever find yourself needing to send out email to your users, the "spam" and notification procs are there for you.
Other aD utility procs make building db-backed pages much simpler than raw AOLServer, like ad_header (which you will probably end up modifying to give your pages a more uniform feel, but it's a good start), set_form_variables, set_variables_after_query, and all the other db procs like database_to_tcl_string. (Alternatively you can use the much nicer 4.x DB api that I backported to OpenACS 3.2, but which wasn't included in 3.2.5.)
Then you have the prototype builder which can be a huge timesaver in getting rough drafts of your pages done.
I'm sure there's more but this is what occurs to me off the top of my head...
