Endless exploration and questing.
Morrowind is a massive open world you are free to wander freely and and at will. It would take hours upon hours to explore the entire game. You came play this game for days and still have an endless amount of content still to explore. While this is one of the many things that make this game great it also work in the opposite way as well. There are fast travel spots at most of the major cities, and you can't fast travel from any were. You usually have to buy a ride or you can get a few teleport spells. You therefor will spend a lot of time walking across the terrain which can become a little tedious over a long amount of time. But the looks of the environment, endless dungeons, wandering monsters and bandits to occupy you while treading the long roads and woods of Morrowind.
You will also find a great many side quests and people you can help while traveling. Along with the open world and endless exploration you also have a very long and interesting main quest line as well as numerous guild quest lines. Though the role playing in Morrowind isn't as in depth in the ways of choice making like the Bioware RPGs such as Dragon Age and Mass Effect, in Morrowind you can pick up anything, steal anything, kill anyone, pick any lock, and speak with anyone. Though you can also get caught.
Sadly along with the truly open world you can easily break a quest line by finding a quest item in a dungeon and sell it. Hours later someone will ask you to retrieve it. By then you will have probably have forgotten who you sold it to. You can also accidentally kill a quest character. If you kill a quest character you will be advised to reload, but as for the guild quests you will just have to quit your life as a mass murder if you want to keep from breaking a quest line.
While doing the major quests you will come across some choices that will add to the replay value of the game. You will also be confronted with quests that you may not carry out if you want to roleplay that in depth.
If you have played Oblivion and Skyrim you will be amazed by the length and size of Morrowind. From the length of the quest lines to the size of the world they generally get shorter and smaller each game.
Along with the length of the main quest line in my opinion is the best. It is by far the most interesting. Though Oblivion's is probably just as good, but as I said shorter.
The environment and architecture is also the most unique. You will find some traditional castles, Victorian style buildings, buildings and towers grown my sorcerers, and shacks and huts.
And to clock out all I've got to say is if you spen the un-godly amount of time to finish EVERYTHING in the game you can get two expansions along with player created content.