Used to express that something is deeply lacking oomph, kick or spunk (Am. definition); to express that something is far from perfect, needs repair, fixing, rectifying. Adversely, the rarely heard "doesn't need any more cowbell" is a compliment of the highest order.
Needs more cowbell! or More cowbell! is yet another one of those pesky memes you either hate or love. It is widespread on the Internet, but not all know what the heck it's all about. Its origins are in a skit in Saturday Night Live revolving around a fictitious representation of the history of the real band Blue Öyster Cult, called "Blue Öyster Cult: Behind the Music", a parody of the VH1 show Behind the Music.
The sketch that spawned Needs more cowbell! is all the more popular, because in addition to the usual suspects, Christopher Walken makes a guest appearance as "famous producer Bruce Dickinson". No, not that Bruce Dickinson, the skit takes place in 1976 and no way can a famed producer be 18 years old. Either a product of imagination or a not-so-famous Bruce Dickinson. Both are probably the case - read on.
The following will transcribe the sketch as seen in SNL. A full, direct transcription can be found here: http://planetwill.jt.org/media/transcripts/boc.html
If you want to see it yourself, a copy can often be found on P2P networks with search terms 'Christopher Walken' and/or 'cowbell'. A copy in audio can be acquired here: http://www.geekspeakweekly.com/cowbell/#
The site also has a few songs that have some cowbell added to them. Note that any copies you find on the Internet may be of dubious legality.
The sketch opens with the members of the BOC in a late night recording session at Sunshine Studios in 1976. Bruce Dickinson appears, commends them on their "dynamite sound", and the band starts to record one of their would-be hits, (Don't Fear) The Reaper.
The lads play the intro, and as soon as the actual song starts, a band member called Gene Frenkle (a fictitious character played by none other than Will Ferrell) starts to furiously pound with a stick at a... cowbell.
The lead singer stops the music and asks Dickinson to come in. "That was going to be a great track, guys, what's the deal?" says the producer.
"Are you sure that was sounding okay?" asks the guitarist and singer, Eric Bloom.
"Honest, fellas, it was sounding great, but... I could have used a little more cowbell."
They have at it again, with Dickinson's instruction for Gene to really explore the space of the studio this time. Frenkle goes at it again, this time swirling amidst the playing band members, inevitably wreaking havoc. Bloom cuts it off, finding Gene's playing distracting. Dickinson is amazed, he thought it was even better this time.
Frenkle offers to "pull it back a little". Dickinson replies, "but not too much, because fellas, you gonna want that cowbell!"
The third time, Gene starts to beat his cowbell completely out of tempo and stares piercingly at Bloom, and then finally kicks the microphone away from him. The members engage in a brief scuffle; Dickinson comes back in. "That... that doesn't work for me. I gotta have more cowbell!"
The members lambast Gene for being selfish. Gene makes a pleading statement by saying how he really has to be selfish, because "if Bruce Dickinson wants more cowbell, we should probably give him more cowbell. The last time I checked, we didn't have much songs that featured the cowbell."
"Give it to me baby! I gotta have more cowbell!" agrees Dickinson.
"And I'd be doing myself a disservice, and everyone in this band, if I didn't perform the hell out of this!"
"Guess what. I gotta fever, and the only prescription, is more cowbell!"
"Maybe I'll just go here and we can lay over the cowbell later -"
The singer interrupts: why don't we lay over the cowbell here, together. Frenkle stares at the members. "Do you mean that Eric?"
"He speaks for all of us." "Thank you."
Dickinson notes: "Babies, before we're done here, y'all be wearing gold-plated diapers."
"What does that mean?" asks one of the band.
"Never question Bruce Dickinson! Roll it!" The producer exits, and the Blue Öyster Cult plays once again, united, with more cowbell. The phrase "needs more cowbell" is never actually said in the skit, though, it is living a life on its own without being in the original source the same way Zulus, thousands of them! is. Thanks to rootbeer277 for pointing this out.
Why is it so successful? It's funny, it has Christopher Walken, and "needs more cowbell" can be applied to almost any situation. It's an inside joke, you'll only get it if you've seen the skit or read an explanation of it, and we all love inside jokes, given we're the ones inside.
Aside the cowbell remixes available in the website already mentioned, the sketch has also inspired a techno rendition with parts of the sketch and an actual cowbell by Dirty Cringos called Cowbell (Main Mix) or just Cowbell.
In case you need it explained to you: cowbell that loud has absolutely no place in (Don't Fear) The Reaper. That and the famous producer ushering them to use it even more is the source of funny, plus the great performance of Christopher Walken, which certainly does not need more cowbell. | 
