I've recently found myself playing a completely new style of music, and one which is typically played by people half my age. That meant I had to do a little thinking about how to avoid looking like the boring old guy who doesn't belong there. That led to a general list of things that guys shouldn't wear on stage, or in many other situations where people will be looking at you and you're hoping they don't find you boring, such as clubbing. A few of them also apply to women, but only a few.

Hawaiian shirts
A lot of old and boring guys desperate to make people think they're not old and boring yet will grab the most colorful thing in their closet. All too often that is a Hawaiian shirt (or even worse one of those short-sleeved shirts with flames printed on it). Either instantly identifies you as an old and boring guy desperate to make people think you're not old and boring yet. People who aren't old and boring just don't have such things in their closets.
How it might work: It's part of some ridiculously incoherent getup - say you wear a basketball jersey over it, a cowboy hat, and tight purple jeans. Then you'll just look like a complete idiot.

Hard-to-see patterns
With distant stages and dim lighting, small or low-contrast patterns become practically invisible. I've definitely got too many shirts like this. Even a really loud but small plaid will just look like a blurry blob from the back. It won't really look bad, but what's the point? Large, high-contrast patterns are better.
How it might work: If it looks OK as a blob but really, really ridiculous up close. Some redneck-styled animal print shirts would qualify.

Brimmed hats
Even if you're young, as long as you're playing anything other than country any hat other than a ski cap or baseball cap makes people suspect you're balding and trying to hide it.
How it might work: You look like Indiana Jones' or Crocodile Dundee's more badass cousin.

Khaki trousers
Nothing says "I'm a boring guy with a boring job I hate" like trousers in any kind of khaki, tan or beige color. Most people just have that association in their mind, and there's no good reason for you to trigger it. Any other color is OK, though.
How it might work: If you're built well enough to look like a male stripper, are wearing nice dress shoes and no shirt, and are playing at a gay bar.

Shirt tucked into trousers
The only people who do this are boring old guys and young guys who are forced to by their job's dress code.
How it might work: You're wearing a suit or at least a sportscoat, and don't have the slightest hint of a gut. Generally the less casually you're dressed the more likely it is you can get away with this.

Leather trousers
Leather trousers might have been "rock'n'roll" once upon a time, but that was decades ago. I was going to write that only middle-aged women wear these nowadays, but then I realized that some homeless bums also wear them in the winter.
How it might work: You're constantly drunk, frequently piss yourself on stage, and want to be able to do it without people seeing anything.

Anything that makes you sweat a lot
Most people work up a pretty good sweat on stage, so plan accordingly. In other words, don't wear that tweed jacket unless you're playing outdoors or in an unheated church in the middle of winter.
How it might work: You've got a suit that needs a trip to the cleaners anyway and don't mind soaking it thoroughly in sweat.

Everyone all in black
It's boring, stereotypical and lazy. It's really a way for everyone in the band to half-assedly match without having to make any thought or effort.
How it might work: It's an amateur performance of classical music where looking boring is definitely better than attenition-whoring.

Looking like the average guy in the audience
The audience wants to think you're better than them in some way... otherwise why would you be worth paying any special attention to?
How it might work: If you look like the average guy in the audience but in a much better version. If your audience wears shorts and T-shirts you can do it too if you're taller, more muscular, and your clothes fit better.

I'm sure there could be countless other items added to the list, but this is a good list of stupid stuff I commonly see guys in bands do. What do you often see on stages that you wish people would stop doing?