One of the most common concerns I saw raised after the announcement of Windows Live Desktop Mail was that Microsoft now had too many new email clients coming out and that this was bad news for users. (Vista's Windows Mail, Outlook 2007 (as part of Office 2007) and now WL Mail Desktop.) When I raised this with the team they said they would address it shortly on their blog, and true to their word they did. The comparison table doesn't include Outlook 2007 but as that is aimed at a different target market to Desktop Mail we'll let them off.
The most obvious conclusion that can be drawn is just how much of Windows Live the new Desktop Mail client will support; from Live Mail synching and Live Contacts integration to Spaces Blog It! on RSS items. What the team hasn't mentioned yet is direct integration with WL Messenger and WL Call, allowing users to start an IM conversation or voice call from within the client. I'd put that under the "More…" category in their feature table :-)
Of course Desktop Mail also includes the consistent UI that is becoming a major part of integrating the Windows Live services. In the future it is likely that your choice of colouring will actually follow you from service to service, something that will undoubtably be popular with the "normal users."
So why have Windows Mail and Desktop Mail? Perhaps given all the anti-trust suits Microsoft has found itself facing recently, it was decided that they could only ship a basic email client with Vista, with the majority of the "Live" extras available via Desktop Mail. Regardless of the reasoning, this will please the Windows users who choose not to use the Live services as now they won't have yet another application to uninstall (MSN Explorer in XP anybody?) Will consumers get confused by the choice available, maybe, though this is where I think the Live moniker will help tremendously. Brandon is impressed though, and that takes some doing.
The new Desktop Mail client is also giving some hope to the Premium users who still use the now discontinued MSN Client. No this hasn't been made official yet by Microsoft, but you can join the dots as well as I can. I would expect the new Premium "Client" to consist of Desktop Mail and Windows Live Toolbar for IE7, especially given that Phil Holden already confirmed Premium users will not be seeing advertisements in Desktop Mail. What Premium users will gain from Windows Live more generally though is still to be established.
The team also blogged about some of the new RSS features in Desktop Mail, the screenshot of emailing direct from RSS is included below. You can read more over at their Space.