I really hadn't paid much attention to these issues until a couple of years ago when I got a 5x7 camera and started hanging around large format stuff and people, when I was introduced to a whole new world of old lenses. There's a big sector of interest there in soft focus, and the lenses that do that, and I learned a lot about it that explained some things in this discussion to me.
The early SF lenses depended on both chromatic and spherical aberration. The interesting thing to me was that these result in two things: first, there's a sense of wider depth of field because with light rays of different colors or from different places on the lens focusing at different distances to the film the effect is that each specific distance in the DOF range has both out of and in focus components. That is, each detail you might look at is imaged partly in focus and partly out, over a wider range, rather than at just one specific distance. This is where the slight halo around highlights comes from, as a blurry out-of-focus bleed and that halo is the second effect on top of DOF. PeterM's shot of the singer on the first page is a really nice faking of the effect.
The other thing to know is that these effects are large aperture effects---stop down any lens and they go away. LF portraitists used this to vary the SF effect according to subject.
As many noted above, with improved computer design, many of these effects gradually disappeared and lenses became increasingly clinically sharp at wider and wider openings. Coming from 35mm, I had always thought that old lenses' lack of sharpness at wide openings was a defect, but now I understand it differently, as a way of adding a certain type of diffusion to the situation, as so many people above have noticed and prefer! So I'm slow to the party here. The other component of it is age and fogging, and in following up this topic more actively, I've found people complaining that when they send their old lenses in for cleaning the glow goes away. :-(
Anyway, modern lenses gradually "perfected" and all of this went away. In the large format world I've personally centered my buying on Tessar types, which have a special (what Leica people would call) glow when used wider, without being unsharp.
By the way, if you want to see some people who are handling the extreme of this effect really nicely, go check the Monocle (single element lens) Flickr group!