After the first couple of days, I was completely unable to complete the Dark Iron Dwarf quest, even at peak times on my server. I don't know if fewer people were participating due to the mounts not being purchaseable by tokens or the non-intuitive way you fight those evil brew thieves, but there were never enough people chugging and chucking along with me to defeat them. Still, that wasn't my least favorite part.
I would have to say that the part of this year's Brewfest that sucked my fun, frustrated the heck out of me and made me only earn enough to join the Brew of the Month Club was the same thing I disliked most last year. It's that apple barrel that is near the guy with the kegs outside of Orgrimmar. No matter how diligent I was doing the Keg Run, no matter how carefully I timed my rein-clicking and no matter how much I jumped on, stomped on or ran around that barrel, it only refreshed my mount about 70 percent of the time. Riding an exhausted ram is so not fun.
What was your least favorite part of this year's Brewfest?