Microsoft is obviously targeting a different audience than they targeted with Windows Mobile. On average, WinMo users enjoyed the similarities of the phone with the Windows desktop platform. Apple showed though with the iPhone that isn't what the world at large wanted. While iOS has some eye candy that is instantly recognizable as Apple, it was an OS all of its own, not trying to be a mini-Mac. Microsoft is taking the same approach with WP7. In doing so, a number of features are missing in what some at Microsoft are calling version 1. Some may be gone for quite some time.
One of the biggest features missing is copy and paste functionality, but we've already been told it is coming, hopefully as a minor update to WP7 rather than having to wait for WP8.
DotNet Reference has a list of may of the features that will most affect developers. We know there is no native code support, and that has impacted the decisions of some current WinMo developers, electing to initially sit out the WP7 release.
Side-loading of apps is not supported, which means 100% of the programs users download and run will be only through the Marketplace, just like Apple's iPhone, jailbreaking notwithstanding.
There is also no traditional file system. This is a huge shift for WinMo customers that are used to grouping files in a similar arrangement they have on their desktop, keeping data in Business, Personal and other similar folders. All file management will be limited to what you see through an individual app's interface.
Despite the platform's heavy reliance on Silverlight, you won't be able to use Silverlight in the browser, which will either break or limit functionality of certain websites, including Microsoft managed sites.
DotNet Reference has others listed. The question is, will all of this matter to the target customer? Three years ago, I'd say no. Apple showed that to be true. Many of the above limitations, or design decisions, are still there for Apple and will be for some time to come. Others, like copy and paste or multitasking have been implemented to one degree or the other, so in some areas Microsoft has some catching up to do.
Is WP7 missing too much, or will most people not care, knowing that the popular features like email, social network, music, third party application store and web browsing will all be there?