In Beta a lot of people get focused on the bugs and "MY DPS IS NOT HIGH ENOUGH!!!!!!!!!" x is better than y ish stuff but you also need feedback on just plain old fashioned playability; what are minor quirks in the game that you think need to be addressed. For example:
I noticed that we've got all this new stuff around the Eastern Earthshrine in Stormwind (pet battle trainer and Panda village/Monk trainer), but the area hadn't been tied into the city all that well for the low level characters who'd now need to get there.
One suggestion later...:
So, don't just be focused on what's "broke" - if you see something minor that just needs tweaking make your case clearly and it might just get done. But remember, the crowd, or Blizzard, could have a different opinion than you so be prepared to defend it clearly and without devolving to insult spam either.