If there is one mistake that you know I will make with Facebook as a noobie.. what would you do different if you were starting today?
One mistake: not measure :-)
Lack of patience.
Get overly attached to a campaign
Spending too much early on - because lower budgets does NOT mean worse results, however it will be slower. Often FB will spend lower budgets (optimized for CV etc) more carefully than burning a big number.
Underestimate the appearance of an ad 
Keep it going for too long before cutting it off and moving on to the next one
Not following the ad guidelines strictly. Wasted a lot of time on mistakes and redo's.