In my research for getting Banshee to compile for Windows, I stumbled on this post. It seems there still lives a LOT of suspicion towards Mono in the Linux community for being a timebomb Microsoft has put under FOSS.
A few days back I read Microsoft released their Community Promise as to defuse these fears. Yet it seems not to be worth anything to developers.
I, as a C# developer, would love it to see my skills go cross platform. What exactly are the arguments against the Mono platform?
It'll always be lagging behind?
Don't get me wrong I love the idea of mono, just trying to think of possible arguments against
Microsoft's track record with respect to open source and competitors in general, is one of the strongest arguments against Mono, due to Microsoft patents.
Microsoft has promised not to sue, but the patents are still there, and there might be other ways for them to be activated.
The Wikipedia article pretty much explains the current legal situation. See the Microsoft Community Promise for how much it covers. So from a legal point of view, you should be save using Mono but you can still be sued for technology you develop (as usual).
On the positive side, developing with Mono should make your code usable for more people.
On the negative side, Mono will always lag behind .NET. Mono will never be 100% compatible ,which might bite you in areas where it doesn't lag behind. Mono also has a bad reputation within the Linux community (i.e. they're labelled as "traitors" bringing Evil(TM) to the free world).
It sounds pretty good for the parts of mono covered by the specifications mentioned. It does boil down to "We won't sue you." It doesn't mean they won't compete with you nor slander you as ingenuine. Also it doesn't promise what other licensees of their patents might do, though I think they wouldn't have much of a case given the promise.
It clearly states that if they get sued, they no longer abide to the promise for the party that is suing them. So if someone invented something patentable that is under the specs, and they patent it, Microsoft can start using it without having to worry about getting sued for patent infringement, because if they do, they can similarly sue. It's actually a lot friendlier than it sounds.
Basically the promise is contingent on a mutual agreement not to sue.
The argument against Mono is based on a notion that using C# will make your code fall under Microsoft patents, but using something else will avoid Microsoft patents. In my IANAL opinion, there's no reason to believe that avoiding C# will decrease the likelihood of infringement.
Therefore, it seems to me that using Mono is safer than not using it, since at least C# is explicitly covered by the Community Promise. Any other alternative technology is covered by nothing, and therefore is more at risk of liability.