I'm a (passive) member of the Tampa IxDA Facebook group and a great question was posed yesterday, " Whats the visual equivalent of cowbell?"
As I see it, "cowbell" is a tool or technique that a well-meaning project stakeholder insists on using beyond the original creator's intent. (That's a really wordy and polite way of saying that it's a thing people in charge abuse the shit out of.) Sometimes creators will anticipate the stakeholder request and work more "cowbell" into their work just to ensure approval.
Either way, it leads to the misuse of a tool that detracts from overall quality and I can think of at least two equivalents in visual design; Gradients and Pie Charts:
I love gradients. I think they're a fantastic tool in the designer's arsenal -- they can create visual depth, draw the eye in or out of an element, create texture and tone, and can help emphasize other design techniques like letter pressing. On the other hand, I loathe to see 6 disparate gradients in a composition. I'm also not a fan of having gradients shoehorned into a comp to fix a 'flat' design. (Gradients can certainly solve this problem, but you can fix that lack of emphasis without going crazy.)
Pie charts, on the other hand, can go die in a fire. Well, I probably shouldn't say that, but they're used really badly in most cases:
If you need a complex labeling scheme with numerical reinforcement to explain your chart, then you're using the wrong chart.
If you need to convey a single data point, say a percentage, then why are you using a chart at all?
Getting a large round object to work in a small square space is just as hard with a pie chart as it is with a dining room table.
Charts are a fantastic tool and almost always look great, but visually encoding your data shouldn't make it harder to understand the data. Design is intended to solve problems, and I think most instances of pie charts solve the wrong problem. (Aesthetics over function.)
I've found that it's pretty easy to convince someone when gradients are being used badly, but the pie chart thing is a constant struggle. I can't explain it, but people will fight for using pie charts until they're blue in the face -- no matter how many options are presented or articles and statistics provided against their use.
Ultimately, I've got bosses to make happy and a family to feed, so I'm just as guilty as anyone else when it comes to adding more cowbell.