Grey level mobs granting EXP.

I do agree this does feel a little off... but I did some poking around.   The mob is green at 55, so you are only 2 levels in from it being gray.. I found that you will get xp +1, +2, +3  levels above a mob's green to gray change.   Does this sound right to you? 

Nope, once a mob goes grey, there should be zero exp, zero honor, etc.
Spells that reward off mobs that grant exp/honor (drain soul, etc) should also stop working.

This is not a bug, at least not what is documented in the video.
 

Quote

For a given character level, the amount of XP given by lower-level mobs is a linear function of the Mob Level. The amount of experience reaches zero when the difference between the Char Level and Mob Level reaches a certain point. This is called the Zero Difference value, and is given by:
ZD =  5, when Char Level =  1 -  7
ZD =  6, when Char Level =  8 -  9
ZD =  7, when Char Level = 10 - 11
ZD =  8, when Char Level = 12 - 15
ZD =  9, when Char Level = 16 - 19 
ZD = 11, when Char Level = 20 - 29
ZD = 12, when Char Level = 30 - 39
ZD = 13, when Char Level = 40 - 44
ZD = 14, when Char Level = 45 - 49
ZD = 15, when Char Level = 50 - 54
ZD = 16, when Char Level = 55 - 59
ZD = 17, when Char Level = 60 - 79

http://wowwiki.wikia.com/wiki/Formulas:Mob_XP

Above meaning you would not get XP for anything under level 41. This function is implemented correctly, we just checked

Hi samljer,

as you can see in http://wowwiki.wikia.com/wiki/Formulas:Mob_XP , in the zero difference section, is ok for you to receive xp from a 48 lvl creature.
As described in this link, only creature with -16 lvl will not give you xp at this lvl