Interesting thread of on JOS forums.
http://discuss.joelonsoftware.com/default.asp?biz.5.248018.15

Here's my thoughts (also in the forums):

I think GinG has some good points. My product has many OS competitors, but we're doing very well so far. I think alot of it has to do with thinking as "corporate" customers think about software.

1. Making the UI pretty is important, especially if your product is targeted at less geeky types and/or needs approval of a less geeky type. When the front line person who found your product and likes it goes into a meeting to get approval for the money how will your screenshots look up on a screen inside a PPT slide?

2. Support, support, support. While most active OS projects have good support in the forums that's not what corporate users want. They want to know that YOU are going to answer their questions. They want a contact, they want assurance that someone will help them.

3. Create a great marketing site. Most OS software have very poor websites that barely even explain what they do. That's OK, because us geeks fight through that or in most cases know what the software does before we even get to the website. Not the case with non-tech users or general folks looking for software. They want to figure what your software does on the first page or at worst after a (nicely done) tour. And again, make sure the site passes the boss test.

Oh and of course you still need to make a great product! I'm only talking about how to market against OS competitors, but if potential customers download your product and don't like it, nothings going to get you the sale.