Now, you can't just walk into Mordor, and the same applies for challenge modes. You have to be level 90, and you have to have completed Stormstout Brewery at least once on heroic mode. Now, having run Stormstout Brewery a lot of times on normal mode, and a few times on heroic mode, as a healer, I haven't really noticed any big differences. Mostly, the heroic mode tuning seems to be that things which really hurt in normal mode really hurt in heroic mode.
Of course, since I last did the dungeon, it's entirely possible that things could have been changed or added. In the meantime, do check out our walkthrough, 5 things you need to know, and bosses in 5 seconds guides on the Stormstout Brewery if you're planning to check out the challenge mode.
And if you're planning to head straight into the beta with four of your friends after you've read this article, you should know that the barrels are currently bugged. Blizzard are working on a hotfix.