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Diversity: Skyrim took away the diversity in TES creatures. There was very little in the variety of creatures or wildlife. Almost to disappointing levels. Coming from Oblivion to Skyrim was disorienting in this department.
Items were also extremely limited in terms of uniqueness. Especially where weapons are concerned. An MMO can not offer so little in these two categories. They will need to offer far more in terms of itemization. The different continents should also promise more in the way of enemy types.
World: I know people like to place TES games on a level all their own in this department. However what's truly there has been in just about every MMORPG to ever exist. I'd argue there's more purpose to these aspects in an MMO because you're playing with other people. Your crafts, your finds, your place in the world all serve a bigger purpose in an MMO. IN TES games your stuck dealing with automated responses, and hearing the same greeting/goodbye or warning you get from the onset. Trade also serves a much more profound purpose in an MMO. The world will be much more alive.
The main thing that will lack in TES is housing, as there is none. Other than that you will actually have a greater diversity on what to do in the world. The online nature is just a different environment entirely, it opens up so many possibilities that are just not possible in a single-player experience.
All IMO of course
For every minute you are angry , you lose 60 seconds of happiness."-Emerson