Like which ones do you mean?
Many design flaws that were believed to be permanent have been changed by the Code Patch.
eg: expanded map, rain/snow collision
Don't want to put words in his mouth, but I
think he meant more like how the combat system was thought out. Not technical aspects, engine limitations or bugs.
To the OP, everything everybody said. Plus I'd add that Oblivion is better if you want action. Morrowind is better if you want immersion. That is, if you can immerse in a older game. Some people I know (who are great people; old games are just not their cup of tea) need photo-realistic graphics, voice acting, and other modern perks to be able to immerse themselves. If that's your case, you might get dissapointed and frustrated.
If you like a good story where it's not too clear who are the good guys and who are the bad guys, if you like gray areas, depth, religious/political/cultural aspects, weird alien hostile landscapes, excentric characters that don't kiss your feet just because you're the playing character, getting lost and not caring about rushing to the finish line, you're in for a treat.
But anyway, you should try it out.
Trying stuff is good. That's how you discover new stuff and get to know about it.