I think it's pretty obvious that the most talked-about and interesting technical topic with Skype is their much-vaunted new "High Quality Video". That gives a good lead-in to Skype video in general, so that's where we will start.
Basically, if you have the right camera and CPU, a good broadband internet connection, and a little bit of luck, Skype will give you 640x480 resolution at 30 frames per second. This is good resolution and speed, and the difference from the standard 320x240 resolution is noticeable, but in my opinion it is not, as one very silly post in the Skype User Forums termed it, "Break-Through Technology", it is just somewhat improved video.
The downside is that you have to have exactly the equipment that Skype requires - they didn't leave it up to the capability of your equipment to determine if High Quality is possible or not. They made a completely arbitrary and autocratic decision that they will actually check the specific camera and CPU you have, and if you don't have one of only three "anointed" Logitech webcams, and either an Intel Core Duo or AMD Dual Core CPU, you don't get High Quality Video. This can be very frustrating for owners of quite a lot of other webcams which are every bit as capable of producing 640x480 resolution at 30 FPS (some even more capable than the "chosen few" from Logitech). It is even worse, though, for the CPU. If you have a multi-processor system, that doesn't qualify as a "Dual Core", no matter how many processors you have. Even more ridiculous, Intel and AMD Quad Core systems also don't qualify, so you are likewise left out in the cold. It would appear that Skype didn't give this quite enough thought before they implemented it.
There is one other problem with the Skype "High Quality Video" implementation, and that is the "more is not always better" syndrome. Basically, if your equipment and bandwidth meet the specifications, you are going to get High Quality Video, like it or not. Users who have volume-priced internet connections might not want that additional data, or a company that uses Skype might not want their internal network to be flooded with High Quality Video, but there is no way to restrict or disable it. The only solution would be to either replace a nice new (expensive) high-quality webcam with something other than the "chosen ones", or to make audio-only calls.
Setting aside the "High Quality" issues, the Skype video implementation is general is pretty nice. When you start a video call, the incoming video is shown in a window within the Skype main window, and your own outgoing video is also shown in a much smaller window just below the incoming video. There is one small pitfall here - if you are skimpy with screen space, and reduce the size of the Skype main window, it is actually possible to make it so small that the incoming (or even the outgoing) video windows will not fit within it. If this happens, Skype just doesn't show the windows! It doesn't warn you, or complain, or put them in their own windows, it just doesn't bother to show them.
If you move the mouse cursor into the incoming video window, two buttons will appear which say "Video In Window" and "Full Screen Video". Clicking the "Video In Window" button will move the incoming video from within the Skype main window to its own much larger window, and your outgoing video into a window at the bottom left corner of that, also somewhat larger than it was in the Skype main window. Clicking the "Full Screen Video" button will do just what it implies - it takes over the entire screen, however large it is, and stretches the incoming video to fill it. The results of this are generally not pretty, to say the least, unless you are receiving 640x480 High Quality Video, and even then they can be a bit dodgy, depending on the resolution of your monitor (and therefore how much the picture has to be stretched to fill it).
There is one other technical area that I would like to discuss, because it is something that is very strange and unique to Skype. Several Skype users discovered a few months ago that Skype was constantly producing large numbers of Page Faults. The number ranged between 700 and 10,000 (!) Page Faults per second, and they are consistently produced, even when Skype is idle and iconified. The problem was reported in the Skype User Forums, and it was sent directly to Skype Development via their "Jira" bug reporting system. The bug report was immediately closed by Skype Staff, with the comment "this is by design", and the completely ridiculous claim that "10,000 Page Faults per second have no impact on system performance". The debate about this continued in the Skype User Forums for a couple of months, with new bug reports opened by users and immediately closed by Skype Staff. Finally, two determined Skype users armed with nothing more than Windows Process Explorer managed to isolate the specific process thread that was producing the Page Faults, and found that they could stop that thread without having any effect on the operation of Skype. When this was reported in the Skype User Forums, the response from Skype Development was "we've never heard of this before, what are you talking about, many other programs produce about the same number of Page Faults as Skype". When that was debunked, both that they had never heard of it, since there had been several bug reports on it, and that other programs produced comparable Page Faults, Skype Development then suddenly said "we have found the problem and it will be fixed". When pressed for further information about what the problem was and why, because of their prior statements that it was "by design", they changed course again and said it was "an oversight, a thread that was put in for debugging some other problem and then forgotten". Since that time, despite repeated requests there has been no more information forthcoming. The whole episode seems very unsavory, and we are left wondering if Skype Development is inept (it was really a programming error), careless (it was really some debugging code that was "forgotten" and left in the product for at least three or four releases), or something more sinister was going on, and they are more willing to have users think either of the previous things about them than to admit what was really happening. Those who wish to read the entire sordid history of this mess can find it at http://forum.skype.com/index.php?showtopic=98518.
I would be remiss in a Tech Talk about Skype if I didn't mention their support, or rather their lack of support. There is no telephone number to call for Skype Support, regardless of whether you need technical or commercial assistance. Likewise, there is no email address for support. The only way to contact Skype Support is to submit a "Support Request" through their web page, and you are then required to wait a MINIMUM of FOUR DAYS before you get the first response - and that response will only be an acknowledgment that they received your Support Request. It will then take days, weeks or even months before your problem is actually solved, if ever. For technical support problems, such as installation, configuration, questions about using Skype and such, this can be quite irritating. But for commercial problems, such as when you have paid Skype for a service and then discovered that they have decided to cut it off, having to wait four days without being about to make or receive phone calls or whatever can be a very serious matter indeed.
I realize that most of what I have written here sounds rather negative about Skype. I have actually tried to be fair, and to discuss only problems and issues that I have experienced or been involved in myself. I would strongly encourage anyone who is interested in this, or who thinks I have overstated the situation, to have a look at the Skype User Forums (http://forum.skype.com), and see the problems and discussions that are going on there.