Ok, this is what I've seen to date:
Time-Life "Muppet Show" (US, NTSC, Region 1): all shows have had the closing shot of Zoot replaced; all Season 1 shows have the later multiple-archway titles. This isn't something that could be "fixed" - it's the only way the episodes are officially available these days. Picture quality is Ok for NTSC discs.
"Muppet Show" (France, PAL, Region 2): all shows are missing the UK skits and have the closing shot of Zoot replaced. Soundtracks are of course in French; however, the Season 1 shows retain their original opening titles
Picture quality is so-so.
"The Best of The Muppet Show" (UK, PAL, Region 2): two discs, each with two episodes, a 50-minute clips compilation and various other extras. Picture quality is generally good, although the episodes look a little soft compared to the clips. Again, the shows have their closing shots replaced. No Season 1 shows have been released as yet.
"Muppet Family Christmas" (US, NTSC, Region 1): badly edited - see the FAQ on this site for full details of the cuts.
"Muppet Family Christmas" (France, PAL, Region 2): the same as the US release, but converted to PAL. Badly. Do not buy this disc, it's unwatchable :-(
"Emmet Otter's Jug-band Christmas" (US, NTSC, Region 1): slightly re-edited from the original broadcast - again, see the FAQ for details.
"Emmet Otter's Jug-band Christmas" (France, PAL, Region 2): same as the US one, and again, an appalling PAL conversion. Do not buy!
"The Muppet Movie" (US, NTSC, Region 1): anamorphic widescreen.
"The Great Muppet Caper" (US, NTSC, Region 1): anamorphic widescreen.
"The Muppets Take Manhattan" (UK, PAL, Region 2): anamorphic widescreen. The soundtrack appears to be mono, which is odd since I'm pretty sure that a) the film was made with a stereo soundtrack, and b) my copy of the VHS release is stereo...
"The Muppet Christmas Carol" (US, NTSC, Region 1): pan/scan release, which is a shame.
"The Muppet Christmas Carol" (UK, PAL, Region 2): also a pan/scan release, and somewhat cut: Scrooge and Belle's duet of "When Love Is Gone" is missing. This is apparently the theatrical cut of the film, so it's more likely a case of Disney grabbing the wrong copy than a deliberate edit. Stick to the VHS release - at least it's complete
"Muppet Treasure Island" (US, NTSC, Regsion 1): once again, pan/scan :-(
"Muppet Treasure Island" (UK, PAL, Region 2): finally, an anamorphic widescreen release! Yay! Sadly, it's been slightly edited for language (to the tune of 2-3 seconds, so nothing serious), but I gather that this was the version passed for cinema release anyway. Whether the uncut version would now get past the BBFC is anyone's guess, but it's a pity that Disney didn't at least try...
"Muppets From Space" (UK, PAL, Region 2): anamorphic widescreen. Nuff said