Guild websites have 2 important functions - promoting communication within and without the guild. This is achieved generally with having a members and non members section.
For the non members section, having a front page blog is a great way to advertise your guild. It can show your achievements, and highlight the great things about your guild. The most important thing it shows is that your guild is organised and active. Your website is the portal to your guild when not in game. It will be doing all the things that you cant do ingame.
Within the guild, one of the main things a website is for is to get people together and the best way to do that is making sure they have a way to communicate out of game. It is a great place for announcing upcoming guild events, getting people to organise things to do, more especially it is used for keeping track of DKP, putting up a raid calendar (to schedule raids and check peoples availability), a place to hold screenshots and mayhaps even a chat feature.
This article will list the things that are important to people who are not members yet, and are based on my own experience and the feedback I have had over the years about guild web site construction.
Customised Domain Name
Something that shows you are a serious guild is a custom domain name. These days they are not very expensive, and often come with redirection services. If you are not very web savvy, it is almost certain that someone in your guild or someone you know is; ask them for help. If you can get your domain name registered and redirected to your guild site, then do it, even if you are using a bulk guild hosting service.
Having a current recruitment status on your front page is a wonderous thing. It is vitally important that the status be updated regularly (preferably with some kind of date on it). I have received much feedback asking about recruitment statuses in my time. By making it clear that they were current on the page, the number of questions I received on the topic diminished significantly. I would then receive positive feedback that the website was active and up to date.
Having someone(s) who are willing to write about your guild on your front page can be a great way to show the world what life could be like if they were a member. Writing about things guildies say in gchat, adventures you go on, acknowledging achievements, screenshots and things like that can make your guild members feel included. Having more than one contributor is a fantastic thing, but often difficult to find. It can also serve as a history for your guild in years to come.
Some guilds like to include member galleries in their websites, where members can upload their photos and screenshots for other guild members to see. Putting them on your front page with a "random photo of the day" application, or linking them in blog posts can keep the guild active and show everyone what your guild looks like and what you value together. Obviously, separating out private, guild only shots from publically viewable ones should be the order of the day there.
Clearly forums are important, as often application processes are done via forums. Some guilds have a public section to their forums (for example Elitist Jerks has a rather extensive publicly accessible section of their forums) and some don't. Making it clear on your website that you have forums is a good idea, no matter how publicly accessible they are.
Never forget to put on your website a section that describes your guild. Who are you? What do you do? What are your goals? Who are your members? Armory Links? Where did you come from and where are you going? This is your advertisement to the world. "Come read about how great a guild we are!!"
Some guilds like to show off their DKP standings and the latest shinies that guild members have acquired. Personally, I would not do this so I don't have a lot to say about it, but it is a perfectly valid thing to show on your site to promote your guild.
Keep it Simple, Stupid
With all of that information on your front page, try and keep that front page as easy to navigate as you can. Menus and drop downs are a great aid to the new user, highlighting of important things like Applications Sections. Links to important places. Remember that most of the people coming to your site will be new users, try to remember what it was like to be new somewhere - what did you want to know about a guild? Go look at other guild websites and don't be afraid to take their ideas and use them on your own website. Use effective graphics and colour schemes; be consistent within that.