Yoshi-P,
My advice to you right now is to avoid falling into the trap of thinking that dungeons are the be-all-end-all of content for MMOs. The reality is, they aren't.
What I expect to see is more open-ended content, and if we are to use FFXI as an example, I would recommend the Campaign system from Wings of the Goddess, with hamlet defense and other activities.
Another piece of advice is to consider implementing scaling difficulties that can be adjusted by the party. Here's how it would play out for dungeons and other content:
5 Stars: Drops +3 Gear
4 Stars: Drops +2 Gear
3 Stars: Drops +1 Gear
2 Stars: Drops NG Gear
1 Stars: Conventional Drops
This way, players who want to participate in dungeons like Dzemael Darkhold without having to be stressed about battle, do not have to end up suffering an anxiety attack (yes, there are people out there who have this condition) whenever a party member gets incapacitated.
The rule of having content like this is to give players the CHOICE to adjust the difficulty to their preference, rather than offering two dumb-arse retarded options like Rank 50 and Rank 30 dungeon options that offer completely different content experiences.
Many upcoming MMORPGs such as Guild Wars 2 and Star Wars: The Old Republic offer "scaling content", which is content whose difficulties can be scaled to the player's preference.
Want insanely-hard bosses? Max it out to "I want to die". Want to have an easy time, not get stressed out and still get the drop you're after? Set the encounter to "Average".
Plain and simple as that. Just copy+paste the GuildLeve Difficulty Star system to Dungeons and you have a solution for the players that don't handle stress very well.
And to those who would feel inclined to torch my suggestions: From a business standpoint, the goal of an MMO is to be inclusive, not exclusive. Being exclusive drives away users.