The Secret World but you've yet to take the plunge, Funcom has published a new blog post that may push you over the edge.
The piece outlines the basics (one dungeon per play field, five-player groups, etc.), but it also gives a bit of insight into Funcom's encounter design. Gone are traditional waves of time-consuming trash mobs. The devs have instead opted to provide a series of ever more challenging boss fights.
Once you've mastered a dungeon's eccentricities in normal mode (and once you've completed all the Solomon Island and Egypt dungeons), you can access each instance's elite mode. If you're skilled enough to survive elite mode, you can test yourself against Funcom's fearsome guardian. If you survive one of his challenges (of which there are three, one each for the main group roles of healing, tanking, and damage-dealing), you can access nightmare mode dungeons.
Read all about TSW's dungeon options on the official website.