A lot of enterprise-oriented features are omitted (just as they are from the base Windows 8 SKU). Windows RT can't join a domain, be a Remote Desktop host, or boot from VHDs, for example. It also has a few omissions of its own. In particular, it lacks Windows Media Player and Windows Media Center.
Then there are the two major differences. First the big bonus feature: Windows RT comes with four Office 2013 applications. Word, Excel, PowerPoint, and OneNote all ship as an integral part of Windows RT. They're not quite full versions of those applications—they lack macros and certain other extensibility features—but they're very close. These applications run on the desktop, and although Microsoft has attempted to make those applications touch-capable, you'll really want a keyboard and pointing device to use them.
And the big missing feature: Windows RT can't run third-party desktop applications. At all. Office is the only desktop application allowed. The only third-party applications allowed on Surface will be Metro-style applications bought from the Windows Store.
In this way, Microsoft made the problem I alluded to much worse—developers can't even port their applications to run on ARM. At the same time, however, it also provided a platform for the creation of new, touch-friendly applications.
There is one final wrinkle. The Office apps that ship with Windows RT are not licensed for use in any commercial fashion, whether for profit or otherwise. In a practical sense it's hard to see how this would ever be enforced, and it's inevitable that someone somewhere will end up breaking the rules, even if unwittingly. In fact, I daresay it's already happened: I'm sure some of the reviews of the Asus VivoTab RT and Microsoft Surface were written in whole or in part on Word in Windows RT.
So what about ARM's power advantage? That may still exist, but it's nowhere near as clear-cut as it was two years ago. Back then, Intel had nothing even vaguely comparable to the high-end ARM processors. But today it does, with its Medfield smartphone processor and Clover Trail tablet processor, both sold under its Atom brand.
In the time it has taken Microsoft to bring Windows on ARM to market, ARM's once overwhelming battery life advantage has been erased. The ARM CPUs may still have a slight power use edge, but the difference will typically be dwarfed by the power consumption of the screen. The Intel processors, in turn, bring CPU performance that is probably best in class (or close to it), and most importantly of all the ability to run the full version of Windows 8 and existing Windows applications. The hardware could look identical to the user, but if it has Intel inside, the user experience will be quite different.
Pricing still isn't known for all the Atom-powered Windows 8 PCs, but indications are that it won't be far off the price of the ARM machines.
Two years ago starting a project for ARM made sense, but by the time RT was near there was no need as Intel had made such progress on energy efficient CPU's. Microsoft should have killed-off the RT project and launched with the x86 product line. RT tablets will not good for Microsoft or ARM as both brands will be associated with a product that is massively confusing to consumers and hugely limited in functionality.