So you want to be in a serious/high-end raiding guild, eh? But there's this pesky "application process" standing in your way, filling your screen with question upon question that let the guild you're applying to know if they're worth your time.
Besides the basics, what can you do to get in, to make your application really shine?
Understanding your class is of utmost importance. Know your weaknesses, know your strengths, both mechanically and personally. Restoration Druids are obviously strong raid healers; but can you say why and how (the answer is not "spam Rejuv lololol")? Do you know what to do should you need to cover for a tank healer?
You'll often get specific questions about stats and their priority. Do you know your haste caps? Are you reaching them? You don't need to just know that high end Restoration Druids should prioritize Haste to cap, followed by Spellpower, Spirit, Mp5, Int and then Crit, you need to know why. When you want to get into an end-game raiding guild, it isn't enough to just regurgitate, you have to understand what you're saying. Reading Elitist Jerks and then memorizing the data is okay, but it doesn't really do anything for you if you can't apply it naturally and easily.
Have a reason for your glyphs, talents and gems. You may be asked about them. "Because I didn't know where else to put the points" shows a lack of understanding. It is okay to admit you're wrong; it is not okay to make up an answer that everyone can tell is bull. Why? Because you will be called on it.
The best way to show that you understand what you're doing? Throw in extras. If they ask how you would heal for Dreamwalker, give them examples of healing outside of portals and being a portal healer. If there are two strats for a fight, don't just pick one; understand your role in both. If they ask to see your keybinds, go one step further and detail which macros are which so they don't have to ask. If you use a Healbot or Grid equivalent, give them the specific mouse binds.
Possibly the most important part of the application is the general final question. It usually goes along the lines of "anything else you would like to add?" This is your chance to head off any possible questions that you either recognize as weaknesses (or undiscovered strengths) yourself, or that you have seen them ask other applicants. For instance, I applied to a top Alliance guild on the server, and they tend to ask Druid healers if they have a geared offspec during follow-up questioning after the official application is submitted. To save time for both people, I created a quick WoWHead profile of my Moonkin gear and explained my limited experience with it, as well as plans to improve. In the end, the amount of extra information I gave allowed them to almost entirely skip the forum question process and request a vent interview.
Write as much as you need to. Be clear and concise as well as thorough. Utilize everything you know about your class and its role (you know you can increase overall raid DPS by keeping Rejuv/WG on other classes, especially DKs, for Revitalize procs, right?) to impress the people whose guild you want to join. Take your time filling out an application, too. I usually write my application and then come back to it a day later to make sure it all seems right before actually posting it.
Keep all this in mind and, so long as you haven't done something to tarnish your reputation, you stand a good chance of getting into the guild you're looking at.