Reasons against:
- Map is far too small for it. Walking around on foot with moutains everywhere, the world will feel big. Flying from one end to the other in seconds, the world feels tiny.
- Dragons should be an intelligent race, not animals you can ride around on like horses.
I agree with your first point, but disagree on your second. Yes, I do believe that riding dragons would take away the sense of scale you will get from having to traverse or go around mountains. This is a similar reason to why I will not ride a horse until I have both seen most of the map AND played a substantial amount of time.
To address your second point, just because dragons are intelligent creatures does not mean we should not be able to ride them. It has been stated that not all dragons are evil, and this implies that we may team up with the more friendly kind against Alduin. So, dragons as intelligent does not necessarily rule out riding them. Now, in my opinion, I don't think there will be any riding dragons, but I'm just saying, dragon intelligence does not rule it out, especially if you've read any of the Eragon/Inheritance series.
I have read a lot of comments on these forums stating that because dragon riding is in World of Warcraft, we shouldn't be able to in Elder Scrolls. But that's like saying that games should not use ideas from other games. But, I think the question should be whether you like the gameplay mechanics in another game such as WoW. Jobs is one great idea that has been taken from games like these. If developers did not take inspiration from other games, we would not now have jobs in Skyrim.
My opinion on dragon mounts is that we SHOULD NOT have them, because of the world size.