Credit and debit cards clog up the concession lines. 
Depends on whether a particular concession requires you to sign. The concessions in the 300 level don't, and things move reasonably smoothly (except of course when it's a near-sell-out and they don't open enough concessions)...the Scoreboard Walk does require signature, and that does gum up the works a bit.
What's worse: people on supermarket queues who pay with cheques (good thing Nats Park concessions don't accept them).
