My take on the '89-'90 episodes is sort of a mix of what has been discussed here already.
The pluses: excellent cast. I think it's unfair to criticize the '89-'90 casts because they weren't Alasdair, Moose, Lisa, Dougie, Vanessa et al - they were all very good actors, particularly Jill (who had already appeared in "Tommy Tricker and the Stamp Traveler" the previous year) and Chris (particularly since he had no prior acting training). And of course with Les and Abby you could never lose. Also, I like that, along with the excellent slimings and waterings, they brought the pie-in-the-face gag back - pie scenes were plentiful in the first few years of the show, but between '83-'87 had been few and far between. I also liked the homages to the past stars of the series, especially the "Age" episode and having some other scattered on-screen cameos. Plus the return of R. Sole!
The minuses: Despite the slimings and waterings being better, the slime and water scenes were often poorly written, at least in my opinion. There seemed to be a lot of sketches that just ended cold after a sliming or watering; in previous seasons, the back-and-forth banter between the actors post-slime/water had always added something to the scene. This problem isn't unique to '89-'90; in retrospect, I noticed it as far back as 1985. Also, though I have a high tolerance for gross-out humor, I felt a lot of it in '89-'90 crossed the line, particularly the scatological humor, and also I NEVER found the fart jokes funny whatsoever.
And another observation: the character of Mrs. Prevert definitely changed with those two seasons. I love Abby Hagyard and it was great to see her get much more airtime, but Mrs. Prevert was no longer the sweet, flighty and somewhat airheaded character she was in the early years; she was not only a harsh disciplinarian, but could be downright sadistic and abusive, even more so than some of the characters who were created to be sadistic and abusive (such as Nasti). I can't imagine the Mrs. Prevert of 1984 doing what the Mrs. Prevert of 1989 did and locking her child in a cage so as not to disrupt her dinner party. In '89-'90, she seemed to always be angry or pissed off at the kids for some reason or other. She got some great lines and her acting was brilliant as always, but I don't think this change in her character was necessarily for the better.