For one thing, the bottles that used to be scattered all over the place were once low-level potions but were now called Underbelly Elixirs. Drinking one -- and you really should be careful about drinking magical concoctions found discared in the sewers -- can create three known effects. One effect turns the imbiber into a wasp, which is pretty cool except that Dalaran itself is a no-fly zone and instantly zaps the user back into her original form. Using it in the Underbelly lets the player buzz around as a wasp for ten minutes, though.
Another effect will turn the imbiber into a naked Tuskarr with a +50 bonus to Fishing skill. The buff lasts 10 minutes, although it only works in Dalaran... so I'm not quite sure what you can do with +50 Fishing there. The last known buff will say that "You had always heard that this was "The City of Mages" but this is ridiculous!" and display everyone as a Mage in classic Netherwind regalia.
Useless buffs? Pretty much. But they're fun as heck, even though I couldn't test them myself as my Beta client kept crashing every time I went into the Underbelly. I always said I enjoyed what Blizzard was doing to make things interesting for players. It's little touches like these that make me feel that the game is special. I hope the Underbelly grows to be a truly bustling part of Dalaran... the darker flip side of the bright and glorious city. Kind of like the Troll Market in Hellboy II: The Golden Army or the Floating Market in Gaiman's Neverwhere. What's next? Weird foods? Silly arms and equipment? More Griftah goodness? There's so much potential for the Underbelly that I think I got excited for Wrath all over again. Check out dinnerbone's site for more elixir screenshots.