City card and predicting when Supply piles will run low
More supply piles tend to go in 5 or 6 player games.

Cards that synergize with themselves tend to get bought out quickly - like Caravan, Treasure Map, and Laboratory. Extra buys also seem to increase the rate at which Kingdom cards get bought out.

Terminal actions (ones that do not give you +1 Action) tend to get bought less frequently, as each player would only consider 1-2 copies anyway. It seems lower cost cards (2 and 3) aren't bought out very much.  4-5 costs appear to go fastest.

I have noticed City itself is frequently the first one to get bought out if any will, and that immediately boosts all of them. Generally I tend to wait on City until I notice people are grabbing Kingdom cards, but I also generally lose to the guy who bought 5 of them.