Lord of the Rings Online brought a lot to the table, but one of the most popular new features was scaling.
It was a big hit, and the developers are now adding it to the rest of the game in the form of instance scaling. If you're curious about how it will work, Content Designer Joe "jwbarry" Barry has written out a developer diary explaining the system.
One concern was making these instances available to the widest possible number of players, while still maintaining quality, challenging content. Joining instances like you would join a skirmish was another detail to be worked out, as was the overall method of scaling.
The full developer diary provides some great insight on how instance scaling will work, as well as the thought process behind the design -- it's a must-read for anyone interested in trying instance scaling.