I could say a lot of things, but I'm gonna try to boil it down.
What I really miss is the casual feeling it used to have. You'd be visiting a familiar place, full of familiar friends you knew so well, but your visits were always unplanned. You'd just wander along, not in any hurry, not going anywhere in particular, waving to all the friends you saw along the way, not knowing who would and wouldn't happen to be around that day. And you'd never know what was around the next corner---the place was so huge and so full of nooks and crannies. Even sticking to the same path would never result in having quite the same journey twice.
I really miss that.
Today, the visits are very different. You can't just wander around, you're guided. Everything is planned and organized. There's no time to just stop and reflect on the scenery, because you gotta stick to the schedule. Hustle hustle hustle. As a result, you see much less and have a smaller experience.
Many of the individual things I miss can fit somewhere inside what I said above. The variety, the eclecticness, the mishmash quality, the breezy casualness that somehow managed to prevail despite all the research behind it....
And also the greater breathing room that all your friends, human and Muppet alike, had to develop organically and be themselves, much less weighed down by specific guidelines. The longer storylines---and story arcs----afforded them that, as well as the improvisional quality many scenes used to have. Not so much today.
The show used to just be so....big. It was a BIG place to visit, and you could totally get lost inside of there, encountering everything from a talking typewriter to a girl taking her llama to the dentist. But since today we have to stick to the path set by the tour guide, we don't get to see people like that anymore, or have the opportunity to meet new friends just as varied and interesting.