Of course, there are downsides. You have to offer support for those customers that have problems (and there will be a lot of them). When they ask for new features, you have to choose to either deliver or refuse them, each option having its ups and downs. When a new major WordPress version comes along, like 3.5 just did a few days ago, you'll have to make sure your plugins support it, or you'll end up with unhappy existing clients and close to none new ones.
It seems it's common practice for clients that experience problems to give you an admin account on their server (either a simple WordPress account or even FTP/Cpanel access). It sure is faster to troubleshoot this way, but I really believe it's a very dangerous practice and you should be wary about it.
Besides all the customer relationship hassle, you have to make sure your plugin supports (almost) all of the possible scenarios that your customers may be using. Things like compatibility with different themes, different plugins, themes or plugins that do not-so-orthodox things in totally unexpected ways, different version of them, so on and so forth.
Overall, I can say it's an interesting ride but I'm not sure whether it's worth it or not. It really depends on a lot of factors.
P.S. I'll post the marketplace and plugin names upon request. I don't want this to seem as self-advertising. Cheers!
Consider this a request :).
Anybody that purchases it can then go ahead and give it out for free or even use it in their own commercial project. The main Wordpress site could also give it out for free as part of their services.
I steer clear of all open source projects (aside from some libraries) when it comes to commercial apps.
Those that don't just don't mention it, like WooThemes. They're one of those millions-a-year businesses. You won't find GPL or any license terms mentioned on their site or during checkout unless seek out their "Terms and Conditions" page yourself -- a link they hide as much as possible -- black text that doesn't look like a link under the logo in the footer, apart from even the other footer links.
In reality, the licensing issue isn't a big deal. There are over 50 million active WordPress installs -- it's a huge market. If you provide an awesome plugin, and you support it, people will pay you for it, even if they could download it off some warez site and you'd have no recourse to go after them.
Part of this license outlines requirements for derivative works, such as plugins or themes. Derivatives of WordPress code inherit the GPL license. Drupal, which has the same GPL license as WordPress, has an excellent page on licensing as it applies to themes and modules (their word for plugins).
There is some legal grey area regarding what is considered a derivative work, but we feel strongly that plugins and themes are derivative work and thus inherit the GPL license. If you disagree, you might want to consider a non-GPL platform such as Serendipity (BSD license) or Habari (Apache license) instead.