OK, I have the latest Zimbra Desktop Beta 2 and the Tags seemed to have an issue... I am running Win7 64Bit. With the 1st Beta, I could drag my Tags onto the email and then it would add those tags.
Now, when I drag the Tags, it will drag up to the Check box area and then stop. Never reaching the emails themselves. Not sure if this is an issue in Linux/Mac, but with win7 it seems to not stop.