It's not that the new dungeons are bad. They're objectively good, even great at places, with a good sense of the history of the place and call-backs to the classic dungeon.
They just feel kind of short to me. Smaller. Actually smaller, not in terms of the size of the hallways or anything but in terms of how much real estate they cover. And while I often rail against nostalgia, lately that sense of scale has been driving me to run older content not even to gather loot for transmog but just to see it, to look around at the scope and scale of the older dungeons.
Now, I don't want to pretend that these dungeons weren't often hideously irritating to run at the time. Getting a 5-man group all the way through Stratholme back when it was all one big, interconnected burning city full of undead and no one ever wanted to clear both sides wasn't anything but an exercise in learning colorful new metaphors as they spewed from your own mouth. You'd end up amazed you even knew the Basque term for that particular deviancy.
Still, there is something to be said for the epic scale of some of the older 5-man dungeons.