Alternative shells are shells created by third parties meant to replace Explorer.  When you boot up Windows, you have Explorer, it's what provides the Start bar/menu at the bottom and the recycle bin and other desktop icons you are probably familiar with.

Many users argue (and I agree with them) that Explorer is pretty bloated for what it does. And it can be flaky. In response, they've banded together to create a category of Windows customization called alternative shells. Litestep, Hoverdesk, Talisman, and Aston are the 4 most popular ones I'm aware of.

Instead of booting up to Explorer, you boot up to one of these shells instead. Unlike Explorer, these shells allow you to apply "themes" which, in themselves, can radically alter the shell to suit a user's particular requirements.

One problem with themes is that they take up considerable memory and resources, particularly the "fancier" ones. Plus, if the alternative shell's pushing the themes over its core features, it comes into direct competition with a second class of Windows customization - Shell enhancers.  Shell enhancers don't toss out the shell, they extend your existing shell (typically Explorer).  DesktopX, ObjectBar, NeXTStart, SysMetrix and others are popular examples of these.

But all of these have some basic shortcomings in their themes: While the underlying technology for these shells/enhancers seem pretty solid, the problem they run into comes when you try to apply these themes. Most themes are hard coded to the original author's personal setup and don't necessarily apply too well to the wider world.

Let me use Aston Shell as an example. Recently, we had a writer advocating Aston Shell pretty hard. I found him representative of the whole alternative shell advocacy community - that their shells are rock solid ready for prime time but other desktop enhancements are buggy. In his example, Aston is rock solid but programs like WindowBlinds are buggy.  It's almost as if there's a certain blindness to the issues that all these "theme" based shells run into: having to spend considerable effort tweaking themes to work on your setup.

When someone says WindowBlinds is "buggy" they're really saying that they have some program that doesn't work with some feature of WB.  Usually when they run into this it's because they've gone into the WB settings and turned on all the features and then run into some relatively obscure program who is trying to draw its own stuff in a non-standard way. It's a cosmetic thing (i.e. the app won't crash or anything).  When we hear about these apps, we look into them and see if we can work around what they've done. 

But as a practical matter, the average user, even with all the WB features turned on, will not likely run into a program that has problems with it.  By contrast, essentially every user of a "theme" for an alternative shell or shell enhancer is going to have to know how to tweak it which may or may not be beyond the ability of the user.

In reality, as in, the real world, WindowBlinds has millions of users. Millions of long term users who have been using it for years. Besides end users, WindowBlinds has serious corporate users including Microsoft, Nintendo, ATI, nVidia, Good Year, Dell, Alienware, eVGA, and dozens of others who have licensed it for use for their customers.  Why? Because WindowBlinds works.

But like all desktop customization programs, there are features that work more universally than others. When we license WindowBlinds to a corporate customer, we disable features that don't have universal compatibility such as menu skinning, console window skinning, and skinning of non-theme aware apps. In essence, we try to ensure that it's as compatible as the included Windows XP themes which is what these corporate clients want.  For end users and power users, however, all the goodies are in there and it's assumed that if someone has some app that's doing something odd with its menu, for instance, that the user will go into settings and exclude menu skinning in that app (or if they want, just turn that off again since that feature is off by default anyway).

So when I look at alternative shells, I look at them in the same light. In fact, I cut even more slack for them because it's expected that their users are either companies looking for a single customized shell or power users who know what they're doing.

After all, anyone who's used any custom shells or desktop "theme" technologies such as DesktopX know that most users who simply download a "theme" are going to have to do a lot of tweaking to get it working on their system and certain levels of flakiness will just have to be worked around. It's not a big deal.

At least, it's not a big deal until one of them starts nit-picking WindowBlinds which has a far wider adoption rate than an alternative shell. A user of WindowBlinds MAY have to make some adjustments on their system depending on what they run.  By contrast, an alternative shell user WILL have to make adjustments and tweaks on their system on any theme of any complexity. So it's a bit galling when one starts praising the more tweak-prone one and slamming the one that works out of the box for nearly everyone regardless of configuration.

The problems I mention in this screenshot are true of pretty much all the alternative shells and shell environments: Aston, Talisman, Hoverdesk, DesktopX Themes, Litestep, and to a much lesser extent, ObjectBar.

So let's go through some issues in a typical theme (see screenshot):

1) The theme is static. If I change resolutions, nothing rescales.  Compare that to ObjectBar which will rescale and so will DesktopX unless its content simply won't fit. 

2) The start menu is one-size fits all. No most frequently used apps list. So the system doesn't even take account to what I use.  Compare that to ObjectBar which does have an MFU.

3) The themes are hard coded to specific apps. What's worse, there's no genericization support. That is, even if you have MS Office installed, no love because it only supports being targeted to a specific path.  Contrast that to DesktopX and ObjectBar which support genericatization. At least there IF you have the app installed on your system it'll work (unless the theme author screwed up).

4) The Start menu are basically modified skinned windows menus. So they don't have the multiple columns like XP nor do they have the smart menus like Windows 2000 (or ObjectBar).

5) No taskbar grouping so if you're like me and have tons of tasks, it becomes a mess.

6) Most themes have cosmetic glitches.  The advantage of having Explorer as your shell is that skins that skin it tend to blend more naturally. Look closely at my taskbar and you'll see how my tasks look kind of iffy.

7) You STILL end up having to run Explorer anyway because they don't come with any real file navigation. If I want to look at my c:\temp folder, how do I do that? I have to use Explorer and voila, now the Explorer process is back using memory.

Now, I don't want to pick on Aston. Aston is, IMO, the most complete of the alternative shells right now. It has plugin support that's pretty decent. It is polished. It has a good UI. It has good skinning features put in. And you can build a useable alternative shell with it.  And the artistic quality of the themes are quite good. The one I have in that screenshot IS A GOOD THEME.

But in terms of being ready to hand over to the mainstream PC user, not even close. None of the alternative shells are ready for that IMO.  Power users? Definitely.  But I'd argue a power user is better served using Explorer with extensions to it. Mold what you have into something better. Not tear it all down and try to start from scratch. The alternative shell people argue "stability and memory use" as reasons to use their alternative shells. But why? These people running some sort of souped up Commodore 64 get up? Oh no, Explorer is using 30 megs of RAM. 

So am I saying that alternative shells aren't the way to go? NO, I'm just saying that they're not the way to go for the average user -- at present. A true power user who can mold the perfect theme for their own setup could make something truly powerful with it (just like they could with desktop enhancers too).  And given the polish level of Aston vs. its alternative shell competitors, it has the edge.  But at the same time, they're living in a grass house to be throwing rocks at programs that have made it into the mainstream such as WindowBlinds.