What Splug said. Your Application is pretty much the entry point there. Show that you know the class, show that you have initiative on what you need to work on.
Remember, they might be a good player in a bad guild. They don't have the gear because they couldn't get the boss down, and in some cases, because the gear simply didn't drop. Gear doesn't determine how good they are, though it can provide a very loose guideline. There are plenty of people in full T8.5 that couldn't heal/DPS/tank their way out of a paper bag. I've raided with people in Sunwell who had a bunch of T6.5 and were terrible players that couldn't think on their own.
Make sure you use the template supplied by the guild, and keep it simple. Colors might attract interest with some guilds, but it might be an instant deny with others (like mine). Know who you're applying to when you write your application, and set your tone accordingly. Your ability to follow directions to the letter in the guild app show that you're able to follow instructions the first time when you're issued them in a raid.
When you're in the guild as a trial, an exceptional player is more than just a guy who comes prepared. He's the guy that isn't repeatedly dying to environmental damage, and if he makes a mistake, he adjusts and doesn't make the same mistake again. Ask questions if you don't know. Be vocal when you need to be, but don't be annoying.
Also, it's a really good idea not to belittle current members when applying. I don't care if you're better geared than our current prot pally, or if you think our offspec moonkin's 4K DPS is trash. You're not going to earn the respect of any guild that way.
Luck is what happens when preparation meets opportunity. -Seneca
Everyone marvels at a square egg, but only the chicken understands the PAIN.