Mix of both. The killer on the server-side is always stuff like missing WCF features, odd bugs with DI containers, missing API calls, slight behavioural differences etc.

Client-side. No WPF and WinForms is dreadfully incomplete.

It's a no go. If you start on Mono and deploy to .Net I imagine it'd work but no-one realistically does that these days. If they were going to do that, Java would be a better choice for the product.