I don't have any specific episodes that I found to be terrible and then amazing or vice versa beyond those I'm sure were affected by the Herd Mind. For example, I found Feeling Pinkie Keen and Over a Barrel to be pretty fun episodes at first, but after the connotations to religion and North American colonialism were put in my mind I can't stand them. Others that I initially disliked, probably thanks to the ton of criticism I read just as they were released like Mysterious Mare-Do-Well, now years after their release I've grown to like. Or others like Look Before You Sleep which everyone praised at the start, I now find average.
The most disappointing of all changes though is just Season 1 in general. I became a brony off season 1 when there wasn't a season 2, but today I find most of Season 1 to be very boring except, of course, the golden classics like Winter Wrap Up, The Best Night Ever, and others. It's a huge bother for me right now to begin a rewatch since watching Season 1 for me is a significant drag. Perhaps I've just grown so used to it that I find it dull; episodes that I found cute like The Ticket Master and Applebuck Season are now mindwrenchingly boring!