1. E-mail Templates 2.0 - the modules make life so easy if you haven't converted yet
2. Subscription/Preference Center - lower your full unsubscribes and gain insight into what your subscribers want to hear about
3. Tokenize more - the list of things you aren't using tokens for is almost always larger than the list of things you are using tokens for Tokens Overview - Marketo Docs - Product Documentation
4. Program Tags - Use Tags in a Program - Marketo Docs - Product Documentation & Shameless Plug of Salytics Tag Manager (free) Salytics Tag Manager – Marketo LaunchPoint
I have also depending on the situation, seen some great web personalization use cases, Ad Bridge comes in handy, and of course the whole Stack Solutions on Launchpoint.
Thanks for the feedback, Chris! I've just finished updating our subscription/preference center so hopefully that will be useful. We use Email Templates 2.0, but I'll have to take a look at tokenizing more and program tags. Thanks again!