No raiding, Elite Dynamic Events take the place of raids for large group content, but instead of being instanced it's out in the world for everyone scaling up to 100 people. 5 man dungeons is where you'll find the hardest content in the game, at the Fanday back in June a group of 5 people did a level 35 explorable dungeon and 4-5 hours later, the devs helping them out, and even cheating by allowing a 6th player in there were able to finally beat it. I expect them to implement Challenge Missions from GW1 at some point, however it may be difficult considering each dungeon has at least 3 paths and Dynamic Events inside of that make for random encounters.
Only way I could see a Challenge Mission system working is if they were to look at what path a group picked and then take into account the event that spawn during the dungeon run and then take the time you get along with the event variables, then post to a leaderboard. You'd need quite a fair amount of specific leaderboards to account for all of the different possible versions of a dungeon you could see, but it's doable.
http://wiki.guildwars.com/wiki/Challenge_Missions for reference. I'd like to see Challenge Missions put into GW2, it's just a question of will it happen or not.