As I said somewhere in the blog comments, it's very, very difficult to justify buying a card at $2 when a $3 card does it so much better. Each card fulfills some predestined role, but if you ever find yourself in the exact situation that a card is designed for, and you still wouldn't take it, then I'd consider it weak.
Of course, you will buy Cellar, and you will end up needing it, but that's just because no card in Dominion is that bad! When every card is good in some way, you need to find ways in which they "suck". Being severely out-classed by a similarly-priced card is a pretty clear-cut way that it sucks. Caravan is outclassed by Lab, but the price difference is bigger, and Caravan isn't really that much worse. OTOH, the badness of Adventurer is just exemplified with Venture. I do buy Adventurer, and I love the card, but in every situation that I would buy Adventurer, I would buy Venture instead if it's available. That, to me, is about as weak as Dominion cards get.