How was it anything but a failure? It had some nice features, but almost no-one ever used it, it was complex, memory hogging and failed to get simple things (focus, tab order) right. I spent 3 years working with it, and it used 10 times the memory of the equivalent winforms app.
No offence but this (as you probably already know) is a lot like the many many (far too many) whiny comparison threads between WPF and WinForms over the years (no different from the annoying C# vs C++ ones either), which almost always gets resulted in links to http://msdn.microsoft.com/en-us/library/aa970683.aspx or something VS2010 related. The extra features it has are not only overused but are likely the reason why so many people complaint about performance; element collisions, multiple effects on elements, effects created on the CPU, animations, animated layout, using media element, 3D content on the CPU are just some of the 'features' that are badly abused, it was almost like people thought WPF was used to make over pretty UI controls that were no different from game entities then apply collision and other pointless crap to them.
WPF gets considered complex, on other forums this may make sense but on a gaming forum WPFs learning curve is a small fraction of what people would have to learn to make even the simplest game, not that it makes any difference since Windows 8 uses XAML so any complexities would be unavoidable if they moved onto Windows 8 anyway.
As for your 'getting simple things right' comment, I am eager to hear what you were trying to do and how you achieved it. Now I have only been using WPF / SL / WinForms for 2 years (although that doesnt mean much since total years != total hours) and while I use all three for different things, my decision is NEVER based on memory consumption mainly because any visible lag is my own coding error and it would seem all the memory complaints after .Net 4 were to do with visual effects. If anything the only resource drainage is from XAML editor in Visual Studios, something that is non-existent in Blend
Besides if you read up on current performance issues, it is almost always to do with effects, media, animations, ink canvas effects or some other eye-candy thing that typically you wouldnt really want to do on the CPU anyway, all which can and should be done using surface sharing via SharpDX / SlimDX. I think the issue with WPF is MS advertised it as a framework for making over the top pretty apps with animations and 3D effects both of which I would never recommend doing directly from WPF.