The surround sound mixer should avoid mixing for one "sweet spot"
I'm not sure how I feel about this. I feel that a mix wouldn't really have much rear activity if it wasn't targeting SOME sort of sweet spot, because on some systems elements get completely lost if you DON'T sit in the sweet spot. Also, I've not released something in a while so this may not be evident but I'm a huge fan of non-standard placements provided that it works for the song. The problem with non-standard panning and placements is that they tend to ONLY work with a well-defined sweet spot.
Work should be evaluated on two or more distinctly different playback systems
I think the more distinct, the better. This is something I absolutely agree with. But I want to emphasize that one playback system that you should always evaluate on is a low-end surround system. Not a trash-can surround system, like one of those HTIB bundles, but a low-end surround system. I personally would not modify my mix to sound better on satellite speakers, but others may disagree.
To that end, the monitoring system must deliver the full range of audible frequencies, and it must be positioned and calibrated correctly.
...I don't think I'm qualified to comment on this.
I find the "neighborhood-shaking subwoofer" bit quite amusing.
Clubs are becoming an increasingly visible area of multichannel reproduction.
Where??? Forget about the setup, I don't think much club music makes ample use of stereo capabilities...
However, too much reliance on the center channel alone can be problematic due to the fact that the center speaker in many home theater systems is smaller than the main left and right speakers.
Normally I would say no, this can be solved with bass management, but as I have found out through experimentation, some AVRs only bass-manage the fronts, and not the rears or center. So, do not be afraid of putting challenging elements in the center, but be wary of how much is in the center relative to other speakers.
Multichannel bus compression and equalization tools have appeared in recent years which can help the front and rear channels be better integrated into a coherent soundfield.
Multichannel EQ? Yum! Multichannel bus compression? It depends. Sometimes it ends up really mushing up the sound field. Especially limiters, they seem to have more adverse effects in multichannel than stereo.
I don't know why I even rambled about this.