Did I lose the "train" on dungeons?
You can zone into an explorable mode if you haven’t done story mode, as long as some one who has done story mode zones in first. You just can’t create an explorable instance yourself until you’ve done story.
Looking for a group for a dungeon is often the hardest part – especially if you want Story Mode. It has little to offer people who have already done it before, so unless you run into a kind, patient soul (who already has all the dungeon gear they want), you’re pretty much looking for the ever-dwindling portion of the player base that hasn’t completed it yet.
It would be helpful if, instead of the dungeon portal leading directly into the dungeon, it fed into a cross-server Staging Area, like the ones from Guild Wars 1. If it worked like a perpetual overflow and took players from all servers, at the very least, all the people looking for Story Mode would be in one place where they could group up together.
While I think that would be helpful anyway (it would be just as useful for those looking for Explorable Mode groups, after all), even that would sort of just be kicking the can down the road. Long-term, I think Story Mode needs some incentive for players to go back and play it again after completing it. I’m just not really sure what a good incentive would be.