Indeed the lack of Thinklight and changed keyboard layout can be a big issue to someone who relies on these. In my case, for instance, the Thinklight is a non-issue, since I never use it on any laptop, but the keyboard layout would be a very big deal...
Overall, the build quality and materials used in SL are not the same standards as in the more expensive T/X series (they use the same ABS Plastic as the edge). And it does affect the price.
But to be completely fair to the SL series you have to keep in mind the following points:
* Not all Thinkpads have Thinklight (some of the tablets don't)
* Not all Thinkpads have a rollcage (the small ones don't, and the concept itself was introduced in the T60/Z60, not before).
* The LCD on the SL500 flexes less than the 14" T series, that's a fact I've actually verified (although more than the 15" T series with a thicker lid, excluding the new T510).
* The keyboard on the SL500, while having a subpar (IMO) layout, feels more solid than what they used to put on the T400/T500 (when they redesigned the keyboard and removed most of the backplate support, which caused heaps of complaints).
* The SL does have the trackpoint, which to me is the biggest Thinkpad trademark, and one that distinguishes the laptop very well from 95% of the consumer machines.
* Personally I don't object to having someone think of aesthetics as part of the design. While I don't find the blinking red dot appealing, I did find the overall looks of the SL series far more pleasing that the general ugliness of the contemporary T, R and to a lesser extent X series. (talking about T/R400/500 and X200, situation seems to have improved drastically with T410/510).
And also, with all of the shortcomings, one should not forget the fact that the cheap, "sub-par" SL series have an HDMI port. A fully working (audio included) HDMI port, which is much better than the far less ubiquitous (and also crippled - no audio support) DisplayPort they put on the T500 (T400 didn't even have that, only on the dock). So with the SL400/500 you got a machine that could, without any external equipment, be connected to any standard digital LCD / TV system. A pretty big advantage in today's world. With the more expensive T400/500 and even W you got stuck without any ability to output digital audio whatsoever in any form.
And, yes, I am aware of the standard response that "Thinkpads are business machines, bla-bla, HDMI is a consumer standard, yadda, yadda, DisplayPort is far better (it isn't), Thinkpads are not for watching movies, etc.". I just don't buy it. And apparently enough people didn't buy it (some of them actually people who use digital audio for work), that Lenovo has eventually caught up with the industry standard and put a DisplayPort with working audio on the new 410/510 series. It's about time.
So forgive me for this long rant in the defense of the SL series, but I do feel that it is often being overlooked for no good reason. Or more precisely, has been - because the new SL410/SL510 I like much less than the SL400/SL500 (due to the glossy screen, lack of higher resolutions and 16:9 ratio).
Finally, to answer the question:
I would like to know why the SL series is in the "Thinkpad" area of this forum, and yet the EDGE is not ... !?
Short answer - because the administration decided so, and that's what counts. Now as to why they decided so? My guess - the colorful lid and chiclet keyboard was too much of a deviation from the traditional design (the SL is more standard in this sense).