What happened to it? My level 91 vulpera monk’s exp buff got replaced with a mastery stat buff?!
New to Monks, I see.
Well, in the old days once you got to Mists of Pandaria level (85 I believe) the exp buffs are no longer obtainable. (So you couldn’t speed through the Mists leveling content in one day.)
It’s also in MoP when Mastery as a stat starts to exist, so you’ll start to see that on gear and such.
Mastery doesn’t exist prior to MoP.
No I have multiple monks, I just haven’t leveled in a long time on another one.
It would be nice to have the continued exp buff until max level though…
Mastery came in Cata.
Yup. Once you hit 90, the buff is mastery rather than experience. It’s lame, but hey, 10-90 with that buff is fantastic.
I had 9 hours stacked on this character before I brought it up.
Ended up with 4 hours of mastery. Let’s be real though we have it pretty good with both the bonus, mobility and strong skills going up. 90 is treasure hunting (even as someone without flying yet) and 100 is done in a single zone at the moment.
(Plus at 90 we get garrison exp buff in addition to everything else!) 
Plan ahead! You can get up to 24 hours Exp Buff before it turns into Mastery Buff, you can get up to 24 hours Mastery buff before you reach level 100. Once you get to level 100 the Mastery Buffs stop stacking and doing any after that point no longer gives you the Buff.