It's research, really. There's two things I find rather interesting about Rift that I think are invaluable lessons for interesting online game experiences.
1. Rift has a flexible character generation system that retains the appeal of a class based system.
Generally speaking, MMORPG character generation mechanics will fall on one of two sides: they'll either be extremely class based and offer you a very limited amount of customization (e.g. EverQuest 2) or they'll be extremely flexible but lost all balance and cohesion (e.g. Champions Online). Mediums, while they can be found, will generally produce an unsatisfactory result that can be attributed to one side or another.
Rift has found a rather interesting compromise that satisfies both camps fairly well. Playing with the ZAM Soul Calculator will probably be easier than me explaining the individual details.
This is an extremely flexible system. You can customize your Warrior, Mage, Rogue, or Cleric across each of their nine different subclasses by simply choosing three sub classes and then choosing where to invest your points into. On top of that, you can switch between four "roles" between combat on the fly, you can switch back and forth from being a Necromancer/Warlock/Pyromancer to a Chloromancer/Dominator/Archon all you like.
You'd expect this to kill all meaning behind the subclasses. It doesn't because you can only play one role at a time and, by maximum level, you'll only have enough points to fully invest in one and a half sub classes (or spread those points around more).
Further, a great deal of balance is maintained, while there are more effective combinations, the way the abilities are balanced between the classes produces surprisingly few redundant abilities.
In practice, players will have their preferences for which configuration of subclasses beneath their main class (Warrior, Rogue, Cleric, Mage) they want to play, and this determines their "actual" class.
Originality is preserved. Balance is preserved. Character generation flexibility is unprecedented. It's a very good system, and one I should gain a lot of familiarity with.
2. Rift has, if not dynamic content, at least a significant step in the right direction over any other mainstream product.
One thing that's often bothered me about MMORPGs are just how static they usually are. They claim to have a virtual world, but they are in reality merely theme parks where (in popular World of Warcraft style) you are directed from quest hub to quest hub performing a procession of quests.
However, no matter how many players perform those quests, things stay the same. There is no true consequence to the players' actions besides advancing their characters. It's a major immersion breaker. Why pay $15/mo for the key to a virtual world that only changes when the developers put out an expansion?
Rift still has the quest hubs and the dungeon instances, but it also adds a completely separate PvE mechanic in the form of invasions from the titular rifts. When a rift opens, it will close after about an hour or so but the payers have the ability to attack and seal the rifts themselves. While open, invasion forces will periodically be launched from the rift against the surrounding area. If not intercepted by players, these invasions will actually claim the quest hubs, requiring players take them back in order to access the quests or other services (healing, general store) there.
That's just business as usual in Rift. Things get even more interesting when "events" occur. These usually involve a massive opening of rifts and largescale invasion while the players on the map are urged to complete certain objectives. The events usually end with a fight with a particularly nasty boss that may require a full raid (four teams of five players each) to take down.
I say Rift's dynamic content exceeds "any other mainstream product" because there are some considerably less mainstream products that push things further. For example, Quel Solaar, while it has several technical and user friendliness issues that make it hard to recommend, it is significantly more dynamic than Rift.
Still, Rift is a good example of how large-scale mainstream dynamic content is not just a pipe dream - important, considering how many people I've seen who try to argue it is. I believe that mastering dynamic content is mandatory to a goal of an everlasting gobstopper virtual world game.
Overall, I think I've a few things to learn from playing a bit more Rift. However, I anticipate that the end game will likely be the same as we've seen in other MMORPGs, and if that comes to pass then I'll no doubt be back here trying to push the envelope.