Sorry if that sounded harsh.. but I really think WPF is a modern application ui framework done right (right not being 100% perfect before anyone jumps on that)..
One of the biggest things a ui framework does is handling layouts, x and y are just one way of layout.. but how many ui frameworks I've seen where x and y are properties of the children.. their properties in regards to the children.. but the properties are
important only to the parent..
Attached properties are a great concept, because they represent that exact thing.. properties that are in regards to the child, but not actually part of the child.. but instead - its parent.
Sure, .net as pure code isn't capable enough to automatically route this nicely, but the syntax for it isn't THAT bad, and xaml IS capable of routing these to look like properties of the child.
This is just one focus, but theres lots of things that have been done right (in my eyes anyway)..