Also noticed this.
My level 10 druid had the following stats:
--- before 10.1
hp 1488
stam 161
mastery 10.7%
--- after 10.1
hp 947
stam 205
mastery 15.15%
Loss of ~500 hp and I see absolutely no difference in damage due to 5% more mastery
How did you farm 28 ilvl? Auction or do you have a special character? If you have a special character, you can show it, or tell which class is best suited for farma 28 lvl