From the satirical Saturday Night Live sketch featuring Will Ferrell, now used as a way to tell someone when something they did was good, but just not quite right. The phrase "more cowbell" works to express this mild dissatisfaction in a variety of settings.
At a steakhouse "how was your meal?" It was ok, but could have used just a little more cowbell.
"Eh, that ,movie wasn't bad. But really could have used some more cowbell."
"That was a good blowjob honey, but really could have used some more cowbell"