Emergent Gameplay:
Real-world economic interaction: Without a global auction house, I can imagine the "black market" will be even more prevalent here. Using online purchase tools/ auction houses to buy good gear for IRL money. This will be interesting to see how the guild auction houses respond.
Daedric item abuse: If they add an ability to acquire powerful deadric artifacts, and they allow multiple players to retrive such an artifact it could have hilarious consequences.
"My lord, the forces of the Aldmeri Dominion approach our keep, their numbers are in the several dozens, how should we respond?"
"By Azura's Countenance! They outnumber us 3 to 1... bring out the Wabbajack squad"
"But my Lord they have always been so unpredict..."
"Damn it all! We need them now D:< "
*after sparkly effects fly over the battlefield*
"My lord... they are... all turned to chickens..."
"Then tonight we dine on poultry... as long as those damn Nords don't get to it all first...:
"Yes lord we know how much you hate those damned Nords, why'd we ever get in a Pact with them"
"Didn't you read the lore? Desperate times call for desperate measures..."
"But... but.. you've traditionally always been enemies"
"Damn it man! Bring that up on a different thread, that is totally off topic!"
"Yes my lord..."
The villains against all reason:
Assuming there is some storyline content that is included in Cyrodil, such as end-game dungeons etc. perhaps a group unites across factions to prevent other players from entering the dungeon. I can see it now on the horizon, 80 soldiers from all races, wearing the same red tabard of the traitor. None using area-of-effect spells in their ranks, but a unceasing wave of arrows and spells rise from 2/3rds of their ranks obliterating all who dare go near the final dungeon who do not wear the red tabard of the traitor. On their server they will keep all from destroying the forces of their dark lord Malog Bal.
Trading thrones:
Even thoughthey have said multiple times that it will be very difficult to be crowned emperor, I believe some campaigns of Cyrodil will start to work co-cooperatively to unlock the emperor skill line for their top leaders. Perhaps keeping their keeps on the frontier, but daily trading their keeps around the capital city to farm emperor status. They know as soon as one faction is crowned emperor, it is the next factions turn to take the keeps, and the original abandons those keeps and let the next faction walk in. I can picture this being as quick as 15 minutes to an hour per "trade". This is an unlikely scenario, but one that I could totally see happening given how many instances of Cyrodil there likely will be.
That's the ones I had off the top of my head...