I've actually been rewatching it on Netflix for a while now, I'm almost done the final season.
I agree it gets worse as it goes on but it's not as bad as people say it is IMO.
Donna and Eric I never found to be likeable characters. They provide basic plot elements like relationship troubles etc while Red, Kitty, and especially Kelso provide all the laughs and essentially steal the show. As the show gets weaker over time these three characters remain strong IMO.
Eric's year off was very annoying, how he "needed to get out of Point Place" but had no skills or motivation was annoying too. Donnas flaky dedication to feminism was also annoying. Their relationship was a pain to watch. Donna being simultaneously referred to as "the hottest girl in Point Place" and a "lumberjack ogre" was frustrating.
There were some continuity errors, but not as bad as other shows. Overall I thought the storylines from individual seasons meshed together quite well.
Jackie and Kelsos relationship was hilarious, Jackie gets less likeable over time. Hyde gets more likeable as the seasons go on. Fez has his moments but I never understood the hype. Leo remains one of my favorite supporting characters ever. Donnas dad is very unlikeable at first but grows on you over time - the fact that he is so weird but gets all the ladies is hilarious.
Agree with the previous posters who said the panties scene was the best. IMO it's by far the funniest scene in the whole series. I also loved the episode where Leo and the guy from Home Improvement "renovate" the Foremans basement.
All in all not a bad show, good for some laughs.