Lookup and filter on Salesforce account object (parent object)
It would be really helpful if you could lookup across Salesforce objects, at least from contact --> account as this is extremely common to so many B2B marketing workflows. Much of the data lives on the account object, but you're always working with contacts in AP, so you need to be able to query both very easily. Not just with Salesforce selective sync, but also for creating smart lists, triggers, actions, etc.
Currently you have to add a custom formula field onto the contact object in Salesforce to pull down any account level data you need onto the contact. This wastes custom fields in Salesforce and creates a lot of additional complexity.
Examples: create a smart list of all contacts that belong to a paying customer (accounts pay/ subscribe, not contacts)
Example: create a smart list of all contacts that belong to accounts that have spent over $500,000 with us this year.
... I could go on and on :)