It seems Miguel de Icaza, leader of Mono, seems to not like WPF at all. I have seen other quotes about it, but this snippet,

Most of what Microsoft calls .Net Framework 3.0 is due to be implemented in Mono, with the exception of the Windows Presentation Foundation, which de Icaza says is “over-engineered, over-architected, and too large” . Even that may come if market forces require it.

reminded me of his distaste for WPF. I am not sure I really understand his dislike. Ok, I understand the reasons he gives, but I am don't really agree. Is WPF big? Yes. Is it complex? Sort of (there is a lot in there but it all makes sense). Is it to large? Well, on WPF/e will do 90% of what WPF will do and it weighs in at about 2MB and that includes a subset of the CLR, so how big can it really be. I know he is talking about APIs, but seriously, its the successor to WinForms (and a whole lot more) so its going to be big.

To me, I think there are some things behind the scenes that make WPF unattractive to Mono. First, it is a ton of work. The wave of .NET technologies and the pace at which they are coming has got to really make the Mono guys cringe. Just compare the sizes of the development teams and see who has to work harder just to keep up.

Second, WPF can do things that will be very hard to reimplement on Linux. All of that coolness leverages a lot of the DirectX infrastructure for both feature and performance. That is even more work for the Mono guys to do if they want to match both features and performance. Performance is the real tough nut since Mono perf has its share of critics in some areas so I suspect this would be one more area that Mono would struggle.

Lastly, and this is a bit more minor, is tooling. WPF almost requires tools. Yes, some folks are doing some amazing things just hand-coding stuff, but for wide spread adoption you need good tools. Mono's target developer is not exactly a fan of buying tools, so someone has to create them. That is a lot of work since WPF does a lot of things.

Just my two cents and fire from the hip thinking. Of course, I won't get into my thoughts on just being copy cats (I know, they have some GDK+ stuff that is original, but come on).