I usually like to play mobile characters that wear lighter armor, and are able to dodge more often and run faster.
Some games however have every character running the same speed regardless of how much armor they have, which to me takes away the point of playing an agile character. (For the longest time I was furious that a heavy armored warrior in WoW was more mobile than my leather wearing rogue).
In past TES games, the weight of your gear made a difference as to how fast you could move, but this being an MMO I'm not sure if they will carry on this tradition.
So far we know that medium armored(leather type) character will have more stamina so they will be able to sprint longer and dodge more often. Is that the extent of their advantage mobility wise? Has there been any mention of a teleport type skill for light armored magic users? Or will heavy armored warriors be able to keep pace?
If mobility is equal for heavy armored characters, I probably will use heavy armor, as it seems to me to give the best advantages defense wise. I see no real reason not to. (Talking about leveling/PvP)