UPDATE: ANet has added a Dye Preview interface, and new dye colors. Yay! Thank you a million times over, ANet!!!
First: Armor Preview [The Fitting Room]
Wouldn't it be nice to be able to "try on" armor and see what it looks like before you go scavenge the crafting materials it requires and pay tons of money for it? Sure, you can get a very general idea of what a piece of armor would look like by checking out the larger icon that is displayed when you click on it at the crafting menu, but that doesn't tell you what it's going to look like on you. I know many of us would hate to buy a set of armor that looked cool in the armorer's crafting window and then have it turn out to look really hideous on our character. I know not everyone cares about what their character looks like, but many for many players, aesthetics are important. I can especially see it becoming more of an issue as expansions arrive and there are even more types of armor and customizations available.
Current Situation
I have read dozens of posts from people who have farmed their fingers off for an expensive set of armor and then can't stand the way their new armor looks and wish they bought a different set or even kept their old armor, anything to get away from the set they just bought. I've had that happen with non-uber pieces of armor, and even though it was only 700 gold apiece it still made me mad that I spent money on what turned out to be ugly armor. Regardless of how much money you have, imagine spending 75 platinum on a set of something that ...surprise!!... ends up not being what you expected it to look like. People spend their money on the 15k armor pieces and fissure armor as a status symbol, or to look different, so why not know what you're getting into before you click Buy?

The Suggestion:
Even if the Fitting Room character display was just a smaller, simple view of your character, like the view of your character and equipment that's on the Inventory screen right now, that would be a big help. Then you could see what new armor would look like and see how it works with your skin, any tattoos, your haircut, your hair color, your equipped weapon and shield, etc. Of course, this would only work for whatever items the armorer you are visiting has in stock, so if you are in Ascalon you can't try on the Dragon Armor there. Plus, since the items you try on are on the 2D inventory screen and not being put on the 3D model of your character that has to be drawn for everyone in town, it theoretically should not lag the server. I think more players would be excited about buying expensive armor if they knew exactly what it would look like on their character ahead of time and didn't have to worry about throwing their money away.
How to Keep Away the Lag [Fitting Room and Dye Preview]
Add the Fitting Room as a tab at the Armorer. Add the Dye Preview as a tab at the Dye Trader. Charge gold to access the Fitting Room or to play with the Dye Preview. If it's something we have to pay for, people won't go and play with it 40 times a day and lag the server.
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There is now a Dye Preview in the game, but I will leave my original Dye Preview section of the post here for reference.
Next: Dye Previews
Everyone loves dye. It helps you stand out or blend in. There's thousands of ways to wear it. Unfortunately, dye costs a lot. Even if you're only buying 1 vial of dye at a reasonable price (400 gold) it still costs 2 platinum to buy enough of that color of dye for your entire set of armor. Imagine buying expensive dye (1 platinum or more apiece), and having it not mix with another color as expected, or using expensive dye alone and not liking the way it looks on the armor you are wearing. Anger, man. Anger.
The Suggestion:
It would be infinitely helpful to have some sort of Dye Chart built into the game, and even more helpful to be able to use said Dye Chart to preview what an available color would look like on your current equipment. If the character model from the Inventory screen is used, and the color is not changed on the character model until you hit Preview, it should minimize lag. As an example, if you try to make a custom color for your desktop background in Windows, you can click an area on the color chart and drag the cursor around until you get the shade you're looking for; maybe there could be something similar with the in-game dye chart, but with the 4000 or so Guild Wars dye colors instead of 16000 available colors in Windows. If you drag the cursor around and find a nice color, you can Preview it and the dye chart could display it on the armor you select, and list how to create the color you chose. For example: [Current Selection is Blue 408 - Blue+Black,Silver]. Then you go buy all the dyes you need from the Dye trader and pay whatever the market price is. Just another way for players to efficiently spend their money.
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Interface Examples (screenshots)
Here is a picture of the Fitting Room idea. In the example, the warrior has just tried on the Ascalon Cuirass from the armorer.
Here is also a picture of the Dye Preview idea. In the example, our warrior pal likes purple and tried on shade purple 206. If he likes purple 206, it points out that he will need to mix Purple+Black and Purple+Blue to get it. (Note: please don't actually waste any dye doing that, I just used plain purple dye on the armor for the example)