To someone with very little understanding of these technologies, could you give a quick word on what the problem is with Mono? Are we talking about performance, completeness, support or something else?

There are a couple of technical issues with Mono, the biggest ones being completeness and tools integration.

But, above all, the biggest problem is not technical and not Mono itself: it is Xamarin. The money decisions on big companies are made by people that need to tell to non-technical bosses and shareholders why they did what they did. And it is much easier to talk to them about Microsoft than about Xamarin.

Remember the "no one ever lost their job for buying IBM"? It works for MS, now.

No Windows Presentation Foundation support is one thing some would miss. Seriously though, there's a lot of clients out there that would probably not miss the Microsoft branding if they got over the .net label missing. Perhaps not larger enterprise clients that rely on specific MS tools and support, but a lot of smaller businesses could leave it just fine that use it for things like asp.net and switch to apache + mono.

To be fair though, the parent I was replying to did not define in what capacity .net would be replaced and how and taking replies as a black and white answer to a gray area question is a bit factitious (this part was not meant as a directly reply to the parent of this reply).