Most advertising companies have fairly strict guidelines about which sites will be accepted into the network and which sites will not. This is also true with banner exchanges though the rules are less strict than an advertising selling company. There are some rules that generally apply to all advertising companies that you should be aware of.

Time rotated
Nobody is going to want to work with a site which automatically changes the advert on a fixed timer such as using Meta refresh in an iframe to update the advertisements. Even if they do accept it, potential advertisers will not really want it either. They paid for a page view not a page view shared with several other advertisers.

Adult content
Unless it is an adult banner exchange or company, the chances are they are going to say no straight away not only if your site has adult content itself but even if you do nothing more then link to adult content.

Message boards
Some programmes will accept message boards, some wont. Generally your site may not be accepted if it is nothing more than a message board although if you have a message board as part of your site you stand a chance. However none are too keen on accepting them because they have content posted which are not from you.

Lots of advertisements
Most companies will not accept your site if it has more than one pop up or more than one other advert banner on any page. If you tend to fill your pages with adverts its time to rethink your design or sell your own advertisements.