Windows Mobile had several problems and one of the biggest was starting with Windows Mobile 5, phone manufacturers began to tinker with the user interface. Palm was one of the first and its changes were minor, adding a few special features. As time went on though, makers like Samsung and HTC customized the interface so heavily, many users didn't even know they were running Windows Mobile.
Not only was that bad for Microsoft as their branding was being cut out of their own product, it was bad for consumers for several reasons. The UI customizations were sometimes done at a level so deep it affected application compatibility across multiple devices. Consumers also had a sometimes less than optimal experience. Things like custom keyboards for input weren't as rigorously tested as Microsoft would have done so glitches would arise. I've had phones that had a non-Microsoft on-screen keyboard that just stopped working after a while, forcing a soft reset. That is not cool when you are in the middle of an email or trying to do a web search.
HTC wants to take another run at it though with its Sense interface according to Forbes.
"Sense is often described as a user-friendly software overlay, but HTC typically describes it as a user-facing experience that unifies its products, differentiates its brand and benefits HTC device owners. It sits on top of the phone's operating system, which in this case is WP7."
That is the problem. HTC is trying to unify its Android and Windows Phone products with Sense. This is good for HTC, but not much else. Phones have relatively few resources to deal with and I am a firm believer in "less is more" when it comes to what the phone ships with. The more you stick on top of the operating system, the more potential issues you have, from slowing down the device, compatibility issues and even issues that require rebooting to clean up.
Let the hardware makers compete on hardware design and quality, and even by adding apps to the devices that enhance the experience. When it comes to the interface though, I hope Microsoft sticks to its guns, or Windows Phone will be headed down the path that Windows Mobile went down, and we all know what happened to Windows Mobile.