There are many places where it's really busted. For example, in WPF, you could pass a ListView a predicate to filter the visible items in it. That is mysteriously gone in WinRT. System.Xml.XPath is mysteriously gone. The list of omissions I've found is full of things like that. It's a very similar development model to WPF, there's just lots of pieces missing.
That being said, I would encourage anyone looking at developing a windows 8 app to strongly look at the html/javascript API for building apps. There's some oddities there too, but in my experience (which is admittedly limited) they are easier to deal with.
XPath functionality's gone? Hmm.. pretty massive oversight. Microsoft's not one to rest on their laurels when it concerns their cash cow though. I expect this'll all make a reappearance very soon enough.
Also, C# (and .NET) were lacking quite a bit in the early days as well.. now C# is pretty much a kitchen sink language.
That being said, I would encourage anyone looking at developing a windows 8 app to strongly look at the html/javascript API for building apps. There's some oddities there too, but in my experience (which is admittedly limited) they are easier to deal with.