Short and to the point or long and in depth?
With or without graphics? If you prefere graphics, do you judge it's quality?

Short and to the point. I had a Harry Potter RP recently that was pretty detailed in terms of the history (it was set several years after the end of the book and the plot involved different changes in Ministers of Magic, uprising of evil, deportation of werewolves, etc.) - but there was an aspect of the RP that, IMO, made it unique to the other games that were similar in plot. So I used a description of that aspect for my plot, and I think it did very well in terms of hooking people and getting them to look at my site. If you can find an aspect of your site to really advertise, then flaunt it. If your RP has a very general, common plot (like high school or a small town), then being able to show your site is unique within the first few lines is key.

I generally say without graphics unless you use a graphic that you know looks good and other people will appreciate. I used a graphic in my HP plot because it was dreary and captured the mood of the game; however, I've seen some poorly-chosen graphics, like ugly icons, or animated GIFs that only serve to annoy me rather than attract me. If you do use a graphic, get other people's (unbiased) opinion on it, and make sure it's under 400px or so in width - otherwise your ad might get deleted by other boards.

Also, keep in mind that some boards don't allow guests to view images - meaning if you other guests look at your advertisement, they may not see your graphic. Lots of sites have one link to the RP, and that's in the graphic... but if the graphic doesn't show up, then its hard to find the link. So make sure to put more than one link to the site in your advert. :]

I honestly have a very short attention span. So long adverts that look like a novel, I don't even read. It doesn't stop me from joining, but I don't read the plot. I like short ones that get to the point. They need to grab my attention and make me want to join. Saying something like "this board takes place in california. it's a school." isn't going to do it for me. It has to actually have a plot. And graphics are always nice. They make things pretty. I know people say that 'the rp is so much more important then the looks. The writing is what's important! Yes, I know this, but come on people. It's ok to admit that you like looking at nice things. Everyone does. Most people wont join a board if the board looks horrible. It's just not appealing.

I think the aesthetics of a board is the least important part of a board. But it's also really easy to make things look decent since there are awesome people out there making skinsand graphics just for the heck of it. Nowadays, it's easy to make a board look attractive.

As for the ad itself, will skim the shorter ones more often than the longs ones, but I won't go to their site if I'm not impressed. For the longer ones, I kind of get anxious that it's going to be long and pointless, but I at least read the first paragraph. If you don't have my attention, chances are I'm not going to read the rest, let alone click on your link.

Short, most people just skim ads anyway. Graphics are nice, but make sure you have more links that just that picture, so put in a clickable title. Oh, maybe some OOC things, like how long it's been open, level, things like that. Pretty fonts are nice as well, but keep the ad readable aaaand that's a bit off-topic. But yeah, I like graphics, they make me click your link.

Short and with a decent-sized font.

An advertisement is just supposed to give the potential members a brief overview and hook them in in order to come and view the rest of the board. If it is too long--or completely irrelevant--then the site is doing itself a disservice.

Get in, tell me what you need to tell me to grab my interest, give me the link and get out. Let me explore a site on my own without having to read huge amounts of backstory in order to get to the point of it all.

Good point.

I get VERY bored with reading hudge paragraths about the RP. Short and sweet should do. :)

It has to grip me. Whether you use font variation or graphics, or plain good content to do it, I don't care, but if I'm not gripped by it I won't read on.

Short and to the point, just partially describing the general idea of the forum. And people need to stop posting images as links to their forum and leaving no other links at all, because a lot of forums don't let guests view posted images, and so you can't get the link at all unless you're registered/find the ad on a forum that will allow guests to view images.

Sometimes I just copy and paste the forum's plot, add a URL and use that as the ad. It's a lot easier than writing two out. But I like short ads. If they're too long, I get bored, and join without even finding out what the plot is.

Exactly as Panda said. Good adverts are so good that although I've know about her site for over a year and have in fact been to it various times, I didn't decide to join until I saw her ad for it and was suddenly very compelled to join.

I realise that makes this post pretty useless if you're looking to do some field research, but that's the way it is.

I do very much dislike small fonts in ads though.

I think the content of an ad is really important; take the best parts about your site and sell them. If your site has really great graphics, then include a nice image that shows it off (and if you don't, don't bother. That's not the important thing about the site). Or if you've got a really intriguing plot, make sure you feature that hook somehow. Show readers what is different about your site. Right now, AG's ad includes a brief convo about souls between two of the site charries which really says a lot about the plot, some quotes of the nice things members and guests have said about the site, and a general list of site traits for those who don't like reading.

Things that are really irksome in ads are text walls, particularly text walls in tiny fonts. Also, I hate seeing the same format for an ad over and over again. Do something original! That should be the goal of an ad, to stand out, not look like everyone else's site.

Also, make the link easy to find. Another ad pet peeve of mine is having to hunt for the index link. Sometimes the ads don't even include them, and I'm forced to look at a plot or a canon list before I can see the board in general.