The biggest problem with IE10 as far as modern web apps go is its lack of WebGL support. Now we have strong evidence that IE11 will support WebGL.
It is difficult to keep a secret on the web, and perhaps Microsoft isn't too worried about people seeing IE11 before it is ready. A leaked build of Windows "Blue", aka Windows 8.1, also contained an early version of IE11. Web developer François Remy decided to see what it was hiding and found that there were WebGL APIs, but they were non-functional. Rafael Rivera, who writes the Within Windows blog, dug a little deeper and discovered the registry keys that have to be changed to enable WebGL support.
A little more research revealed that the WebGL API works with either HLSL or GLSL shaders depending on a registry key. This strongly suggests, although at the moment nothing is certain, that IE11 supports WebGL via DirectX rather than OpenGL. If so this fits in with Microsoft's long time negative attitude towards OpenGL - Microsoft might be making a play of embracing open software but not at the expense of its own products.
Apparently the API works so well that you can take existing WebGL programs (with OpenGL shaders) and just run them - as proven by this video from François REMY:
Ostensibly Microsoft refused to support WebGL because it made use of, and provided access to, the GPU. This might allow an attacker to execute code that could get to the rest of the system. Other features in IE10 make use of the GPU to speed up 2D graphics, but this is safe. A more rational, though unproven, explanation of Microsoft's rejection of WebGL is that it really was a rejection of OpenGL in favor of DirectX. At the time it seemed that a better strategy, rather than denying IE 3D graphics, would be to implement WebGL directly on DirectX and it seems likely that this is what has happened.
Of course, all of this is subject to change in the final version of IE11 and if they can't get it to work well enough it might even slip to IE12 or be dropped altogether. However, there does seem to be sufficient evidence to conclude that WebGL has a good chance of being included in IE11.
Now we have to wait for some official word from Microsoft as to what is going on. I wonder what spin they will put on rejoining the browser fold?