[Windows Phone 7 General] N8ter:
There aren't that many unique features in the OS, which aren't tied to Microsoft services, which is an issue. Many people don't like Microsoft Services (i.e. Windows Live). I think they're Fabulous, but there have been scandals all the up to 2009 (Hotmail Password Leak, Bad Account Security, etc.) and people are just fed up. Overseas Windows Live is really strong, but lots of Microsoft services aren't there yet (or aren't as good in the US i.e. Zune, Bing Maps, etc.).

Another issue is the phone doesn't have that many Unique features period. It's an Evolutionary OS, not a revolutionary OS. Everything Microsoft tried to market, people looked at their phones from early-mid 2010 and said "Oh, well... I can do that on my Android/iPhone/Symbian/whatever. Even things like Bing Maps which works flawlessly on Windows Mobile, is botched on Windows Phone 7.

Major Marketing points, common rebuttals:

Live Tiles - But they're just Analogous to Widgets on Android (which can be updated via Push Notifications) and Tickers on icons. Most Andorid phones coming out have resizeable Widgets, as well. Most Andorid users don't care about Live Tiles because their Notification system is superior to WP7's by far. Apple is copying Android's Notification system. Microsoft cannot depend on just Nokia and new smartphone owners. They have to siphon off users migrating from other platforms as well - as Android has so successfully done the past 2 year or so...

It's fast - WP7's UI has a lot of dead space, uses a hardware scaler (so graphics are rendered at a low resolution and then scaled up) and tons of text. That facilitates higher performance. Android also didn't have Hardware Accelleration across the OS until Gingerbread (though some OEMs like HTC did a lot of modification to enable it in some areas i.e. Sense Weather Animations on the home screen and things like that). Games generally run better on high end Andorid and iOS devices due to superior hardware.

An interesting tidbid is that the Launch WP7 devies record 720p at 25 FPS, while Galaxy S devices do 720p at 30 FPS. Things like BT 3.0, BT File Transfer, etc. are also missing from WP7 devices (but may be in Second Gen Mango devices).

SD Card Support - A couple of phones can play roulette to add more storage. Most phones don't even have an accessible slots and in most markets customers weren't even given a choice in storage sizes. Dell is the only US WP7 device with (apparently, really good) choice (8/16/32 GB variant of the DVP).

Integrated Social Networks! - This has been common on Smartphones since late 2009. The HD2 had it. All Samsung Galaxy S phones literally had a People Hub (WP7's version almost looks like a carbon copy by comparison). In the Galaxy S II:

Feeds - Can see all together, or filter down to only one Service. Also notice the Message Tab, which shows Messages from most services including SMS, GMail, Email, etc.

Ticker to show you how many Social Updates you have.

Skydrive Integration - Already on Andorid phones via the Preloaded Office applications, which apparently are superior to Office Mobile for Editing and integrate with DropBox and Google Docs. From GSM Arena Review:

No Flash or Silverlight Support.

Notification System in WP7 is on par with iOS4. iOS5 will be on par with Android for Notifications. This is a particular sore point for me. I think their Toast Notifications are awesome. I think Live Tile Notifications are no different than those tickers in Android and iOS. But the lack of a Notification Tab is a really really big issue for me.

Visual Voicemail in Mango : Click on Voicemail Tab on a Galaxy S and it launches the VVM app, which launches in < 1 second.

Integrated FB/WLM Chat - Andorid has Google Talk (with Video Chat) basically built-into the OS. Most users on iOS/Android don't care since they buy clients like Trillian or BeejiveIM to access all the chat networks they use in one application.

You can do group chat in those applications, and you can send more than just photos in some of them as well. WP7 Mango uses MMS to thread group chats (SMS), so some carriers are disabling the MMS because it's expensive in areas where unlimited MMS is not common or where the carrier infrastructure isn't as good. In that case, the group SMS won't be threaded properly. Replies form different contacts will be in their own threads.

I don't think App Count means anything, personally. As long as the big players get on WP7 as soon as they can it won't be an issue. They seem to be jumping aboard, so WP7 is good there.

The biggest issue I see is:

1. Hardware doesn't attract attention (when it does, it's not positive attention).

2. There really isn't any real Flagship phone, and it's almost impossible to have one due to hardware requirements imposed by Microsoft (no one can go above and beyond, except Nokia?)

3. It's not really a revolutionary OS. It's just Evolutionary, and barely that. All it does is take existing functionality and wrap it up in a different user interface/user expeirence.

Whether users choose WP7 over Android and iOS will not depend on functionality, since (esp IRT Android) those OSes do much of that, anyways... It will be because they prefer the WP7 user experience (and UI style) over those competing OSes. It won't be because WP7 hardware is better either, since Microsoft has made that impossible with their strict hardware specs.

Microsoft needs to get WP7 up to feature parity ASAP. The main selling point is not their services or one or two supposed killer features (like Bing Audio). It's the User Experience.

And when what you can do with the phone is so limited, it completely kills that story.

I think Mango is a huge step forward. I think the next year will be pivotal, espcially as Android and iOS update to ICS/iOS5 as there is likely to be a reopening of part of that rift again.

I don't think Windows 8 will entice many users to get WP devices, just as XBox Live didn't entice many of them to buy into the platform.