I'll get things started.
- Scrying Pool is awesome, except when there is no source of +buy / additional gaining.
- Rebuild is like the dumbest card ever, but should almost always be bought as early as possible.
- In Cultist boards, I usually open Silver / Silver so that I can buy one on T3 or T4.
- Moats are twice as good in 3-player games as they are in 2-player games.
- Most Knights don't actually give you anything right now -- so you will need some sort of additional payload if using them.
- Some piles pretty much never ever empty: Chapel, Jack of all Trades, Cutpurse, Outpost, etc.; while others very often empty: Herald, Grand Market, Peddler, etc.