At the core of these works is the idea of "fusing" with a shortwave radio event and then performing considered, disciplined explorations (solo, duo and trio variations) of these basically unpredictable sound events (unless one knows the local DJ and can make requests :) ). It's possible that in POLE and EXPO that multiple layers ("trains") of radio themes could surface, but any parallel tracks usually end up joining due to the frequent instructions to "relate" to another person's layer.
As I wrote in my SPIRAL impressions, I think the impact of these works has much to do with the players and their chosen instrument(s). Stockhausen has arranged some of his other works for alternate instruments (IN FREUNDSCHAFT, TIERKREIS, etc...), and some others also have open-ended instrumentation (STOP, YLEM, SOLO, PLUS-MINUS), but I think none are as open-ended in content as these works, leaving the performers' instrumental timbre and playing style as defining characteristics for each interpretation (even the "intuitive music" works (AUS DEN SIEBEN TAGEN, FÜR KOMMENDE ZEITEN) have at their core the rhythmic concept of slow ensemble "vibrations"). The SPIRAL-POLE-EXPO trio of works seems more concerned with relative degrees and kinds of reflection between individual forces (including with oneself), rather than larger structural processes. Processes do exist, but since the choice and degree of parameters are so open-ended, it's harder to sense these without having an annotated score at hand. In fact, it's even expected that performers start from different assigned points in the score from performance to performance. ZYKLUS and KLAVIERSTÜCK XI also have this kind of "polyvalent form", but those are quite thoroughly-notated for percussion and piano (respectively) and require the player to "finish" the piece in one performance.
Probably more so than in any other Stockhausen work, the performers' individual instrumental style comes through as a primary focus when listening to these works, especially considering that there could be a relatively wide latitude to the interpretation of the score symbols. In other words, it may be possible to consider the version of POLE by Eötvös and Bojé as "showcases" for their skills on electrochord and electronium, and the more recent versions led by Michael Vetter to be something like fast-paced Brecht-ian comic-operas, threaded through with absurdist sound poetry and ethnic folk stylizations. Another version of POLE in 2010 featuring European free improvisors Frank Gratkowski (on saxophone) and Anton Lukoszevieze (cello) sounds very much like, well, European Free Improvisation.
Now, remarking on these performer-based factors is not meant to undercut Stockhausen's contribution, since the key idea of using a shortwave to germinate trains of development is unique, and the "velocity" (or perhaps "proportions of change") in these works is very Stockhausen-ian (and possibly even serially organized). The notation symbols are also a novel way of forming clear dramatic arcs, but leaving room for many surprises at every performance. POLE is also the only work of Stockhausen's which includes a "mix balance" score for live performance (other electronically-realized works of his have notated motions, but those are not really for a performers to interpret live). But for most casual listeners, the appreciation of these works may in the end come down to how much the listener likes the natural improvisational style and instrumentation of the performers themselves. Actually, come to think of it, one could say that these works are Stockhausen's gift to the creative improvising musician.
I feel that it could be useful to have a discussion-forum on the music of Stockhausen. There are so many people from all over the world, young and old, learned and eager to get into contact with this musical world: musicologists, composers, musicians, music lovers; people who plan concerts - who write books or have to give lectures and so on. So there should be much stuff, many ideas that we can share. And when we have open questions, there may be people who studied just that and could give a hint or a stimulus.
A problem might be the English language, but i feel that is the only possibility that many people who are interested can participate. And we can exercise tolerance to mistakes!