I think ChrisA has a valid point from a user's perspective, one that I felt was valid on a lot of previous "feature-packed" OS upgrades.
Consider: Tiger works just fine on one's machine. Leopard (especially wireless) was awful from 10.5.0 until about 10.5.3 (yikes). Even after the bugs were worked out (10.5.3 SHOULD have been 10.5.0), was there an obvious incentive to move from Tiger to Leopard? Depends on which Leopard features one valued.
Now, if the feature set for Leopard only included things like sandboxing, DTrace, and CoreAnimation support throughout, then I think that the value statement is perfectly valid: the merit of these and many other features is not 100% obvious and often invisible for many end users. So many aspects of Leopard benefitted the developer, which is great for end users in the long run. Happier developers with very easy to use tools means that more people are willing to write higher quality applications, which means that Mac is more useful, which means that users are more apt to stick with/switch to Mac. PS. The real value of Leopard was in those dev side features, in my opinion. While a few features such as QL are useful, most of the stuff they were selling (transparent menu bar, reflective 3D dock -- c'mon) was kind of silly.
The things you list, sidewinder, are undoubtedly beneficial to all of us (and I am personally giddy with excitement over SL), but we have to admit those things are pretty tough to sell in an age of already fast computers (faster), huge cheap storage (less space), already efficient (efficient), etc. etc.
Plus it's going to be hard for a lot of end users to justify an upgrade to SL with all the economy stuff going on.
I think it's obvious that it should not be free. I'm already sold that the work gone into it is massive. And we're not talking "things that should have already been in Leopard." We're talking major rewrites of various aspects of the OS from the kernel up.
I'm happy to pay for the upgrade -- but I really don't think that valuation of SL at $129 is going to be feasible without "features" that they can actively sell to their larger-than-ever existing user base, especially in this economy.