The alien touches to races, architecture, society and so on. I really missed that degree of exotic flavour in Oblivion.
Series implies sequels or prequels.
So what? Still a sequel.
Not sure I understand all you say, but usually forums have a seperate forum for each game, being from a series or not, sequel or not. It's to put all the discussion about a subject at the same place, so people don't talk about totally different things. Nothing to do with Skyrim being a standalone game. Skyrim IS a sequel to other ES games, it IS from a series of games, thus we CAN talk about how it fares compared to the other games. We're not comparing Mass Effect to Oblivion here, but a TES game to a TES game.
Cyrodiil is not an exotic place, unlike Vvardenfell. So the plant life and such that grew up in and around a landscape scoured by ash storms, or ancient desert denizens whose husks could be used for dwellings... that was all "of a place" for Morrowind, it made sense there. It would have been
entirely out of place in Cyrodiil. I can understand (though not necessarily agree with) complaints about Oblivion being boring for all that it was a more traditional faux-medieval LotR type setting, but people that think that, for example, Telvanni mushroom towers or Ald'rhun husks as buildings were "missing", have maybe not tried to step back and take a bigger picture of the ES world - which encompasses fare more than Cyrodiil and Vvardenfell. If you could not overcome the dislike of Oblivion's landscape/art design, that's fine. But saying the exoticness of Morrowind was "missing" shows a lack of understanding of where or how Cyrodiil fits into its world. imo, of course.
I loved Morrowind for many reasons, but I don't play The Witcher, or, say Ego Draconis, or Dragon Age Origins, all of which have the faux-medieval type fantasy basis for the art/world design, and sit there going "aww, I wish I could see building made out of bug shells." Or "You know what's missing here? Giant parasol mushrooms and weirdly jointed animals.
As for the sequel vs. series discussion, which still makes me mildly queasy due to Fallout 3 ad nauseum arguments, these are games in the Elder Scrolls series. I could be wrong, but I think Bethesda usually refers to, say, Skyrim as the "follow-up" to Oblivion. They probably purposely don't use sequel because sequel is a string of related events. A sequel would pick up shortly after the prior story left off. Again, i will use The Witcher - the 2nd game coming out in May is a sequel - the events of the prior game will be acknowledged in some manner in the current game, as the story of Geralt continues.
Having blah blah blah'd all that, I still love Morrowind. I still love Oblivion. I suspect I am going to love Skryim and lose another 1000 hours over time.
I have never felt the need to compare them, because each game is a standalone story. With good decisions and not so good ones - which would be true of every single game I play.