Well, I'll try to give my answer to the questions you asked, in the order you asked them.
I'd say the Tilt 2 is a bit more "mainstream" a device in that its specifications aren't exactly breathtaking. It can be argued that this is where the device derives much of its consumer strength, but I think it more likely that the "wow factor," to borrow the regnant colloquialism, is much more pronounced in the HTC HD2. The Tilt 2 is bulky, to be sure; its heft is well known among Touch Pro 2 fans worldwide. The lack of a 3.5mm headphone jack is sure to leave a large number of casual users wanting, as is its terrible multimedia capabilities. The chipset is just about as dated and slow as the HD2's is modern and fast. This is simply not a future-ready device. There's no doubt that such an underpowered device is completely Windows Phone 7-incapable, at least going by the rumored high cutoff for the recently-announced upgrade to the OS family. The HD2, on the other hand, is oozing with features and style. The lack of physical keyboard will leave some business-oriented consumers wanting, but the style, screen appeal, power, and performance of the latter device have proven its greatest strengths. Perhaps a rule-of-thumb is in order to better characterize the distinct demographics favoring one device over another: those aware of the differences in specifications and capabilities of the two devices would almost invariably chose the HD2, unless a hardware keyboard is an absolute necessity to them (or they simply cannot afford the HD2).
As of this moment, Windows Mobile continues to improve. The release of 6.5 was admittedly underwhelming, and arguably doesn't bring to bear the full development speed of which software giant Microsoft is undoubtedly capable. However, refined versions such as 6.5.3 and other "leaked" builds show a great deal of progress being made on the aesthetic points often singled out in criticism of Windows Mobile. HTC's practice of shrouding the hitherto "bare" or "dated" interface of pre-v6.5.3 Windows Mobile was (and perhaps continues to be) a necessary step for them in alleviating these aesthetic issues while providing a somewhat more efficient user interface for the interests of the typical consumer.
It is something of common knowledge within fan circles that Microsoft plans to release an entirely revamped variant of its mobile platform in the coming months. Its strengths will likely be in Office integration, drastically improved user interface extensions, and an updated kernel. It may also offer enhanced connectivity and media sharing abilities, as well as come in two flavors--one catering to the business user, and the other to the media enthusiast. It is a common fear that this new version will even be too different from its predecessors, in that critical software designed for older versions of the platform will be incompatible with the newer design.
I cannot comment about Blackberry or its userbase, as I have no extensive experience with them. Speaking in generalities, however, it may be said that Blackberry users, like some iPhone users I've known, would be at first daunted by the unfamiliar model (hence the existence and continued use of custom themes, as exemplified by HTC's Sense). However, upon further exploration and perhaps "tweaking" of the system, they will soon discover the power of the operating system, which in many ways behaves like that of a desktop computer. The extent of customization and software freedom would likely appear vast to a user of a less desktop-like system. The popularity of windows mobile among tweakers and self-proclaimed geeks (as well as platform enthusiasts) may be partially attributed to these and other abilities of the mobile Windows platform. Entirely new capabilities can be added or unlocked from the phone at a fundamental OS level, rather than being "tacked on" as with an app or plugin. The system can be fundamentally reconfigured for greater performance, lighter resource usage, different visual performance, updated OS versions, unsupported/custom ROMs, and so forth. There have been cases in which newer phones have been obtained by a tweaking community and the software made available to users of other phones, essentially replicating much of the experience of using the new phone on different or older hardware. In one extreme case, the Dell Axim x50v (released circa 2004) has been fitted with Windows Mobile 6.5.3, gaining virtually all core features of the platform without paying for any upgrade. Note that this particular device is competitive or even superior to the much-newer Tilt 2 in terms of graphics, video, and gaming performance.
As dedicated apps (or "programs," as historically termed in Windows Mobile) go, there is a tremendous amount available, especially considering the long history of Windows Mobile development which ramped up around 2002. Many of these programs are fairly extensive in scope and reflect a higher quality than their counterparts on competing platforms; for instance, a file browser program in Windows Mobile would generally be expected to have a large amount of functionality in comparison to a file browser in any other mobile platform, especially considering rival platforms place less emphasis on file structure. This is a reflection of the desktop orientation of the OS. There are admittedly fewer simple apps--any search of the Apple App Store would produce thousands of tiny, cheap, often feature-barren programs of many classes, such as games. Windows mobile also features some of the best emulators for games, no jailbreak required. There is a Playstation emulator in circulation, as well as those for the Gameboy (Advanced), (Super) Nintendo Entertainment System, point-and-click adventure games, and so on. These are often of exceedingly high quality, and are all available for free. Regarding the "high quality" classification I apparently appended to these programs, all of these mentioned emulators function with full performance and minimal frame-skipping on the 2004 device I mentioned previously. The same cannot be said of emulators on competing platforms such as the iPhone, where a "jailbreak" is required to even load unapproved apps--a workaround that is becoming ever more difficult to perform, and as of writing is impossible on the latest iPhones. Blackberry has even less to offer in this regard. Then of course there is the issue of productivity applications. Office Mobile is a fair bit more advanced than the competition, which often struggle to simply display office documents, much less create/edit them, save them, upload them to ftp or a remote server, convert file format, or perform any of the various functions that Windows Mobile users have taken for granted for a long time. Users can install browsers of their choice--Skyfire, Opera Mobile, Fennec, and such are all available for download and use besides Internet Explorer. There is no browser lock-in. Opera and Skyfire even support Flash, which rival platforms struggle with (or ban outright).
I hope the above will prove insightful to those who haven't grasped the power of the Windows Mobile platform. In terms of light usability, it currently ranks among the lowest of the competing mobile operating systems. In terms of aesthetics, it ranks likewise. In terms of "innovation," whatever such a term might imply, there has been little change in its model for years. For users simply wanting to make calls, type texts, or stay in touch by various means (from facebook to messengers), it would suffice, but not in any overly efficient or competitive manner. It is chiefly in the desktop-like power, compatibility, software openness, tweakability, and performance (games, etc) that make this system shine for a still-respectable base of users.