By "rest" I assume you mean Oblivion and Skyrim.
Well, I enjoyed the freedom, the rewarding exploration, the sense of mystery (Red Mountain, the blight disease, the Sixth House, ... what are these things?), the lore, the alien environment, the political intrigue and the tension between the various factions (Empire vs. Dunmer, the three Great Houses, ...), the music, the scary dungeons, the long questlines, the sense of advancement, the many character-customization options... just to mention a few things.
All of these elements have either been completely removed or dumbed down in TES 4 & 5.
Edit:
It was the first one they played. Nostalgia kicks in and they start attributing it qualities it never had.
Oh, give me a [censored] break... I'm getting
really sick of this stupid, baseless "it's just nostalgia" argument that's being used mostly by people who haven't
actually played Morrowind. It is most definitely NOT nostalgia that makes us feel that TES 4 & 5 aren't as good. In fact, I'm playing Morrowind right now and I'm still enjoying the heck out of it, and I find it superior to TES 4 & 5 in so many ways. And there have been various posts (not in this thread, but elsewhere) made by people who've only started playing Morrowind relatively recently that concur with our "nostalgia-blinded" opinions.
Oh and FYI, Morrowind wasn't my "first" TES game either.