From WoWWiki

From developers:

The general resistance data is as follows for PVE: 

Vs. +0 level mob (same level), old resist: 4% new resist: 4% 
Vs. +1 level mob, old resist: 5% new resist: 5% 
Vs. +2 level mob, old resist: 6% new resist: 6% 
Vs. +3 level mob, old resist: 37% new resist: 17% 
Vs. +4 level mob, old resist: 48% new resist: 28% 
Vs. +5 level mob, old resist: 59% new resist: 39% 
Vs. +6 level mob, old resist: 70% new resist: 50% 
… 
And so on (+11% for each additional level)  

Basically this means that against higher level mobs, feign death was 
more likely to fail in the past. If there is a high level mob when you
feign death, there is a better chance that the feign death will be
resisted. 

During controlled testing these tables were confirmed to be correct. 
This isn’t to say that other feign death bugs may be occurring, but 
as for the actual resist formulas, you now have them to compare to. 
If anyone finds data that is off from these tables when testing, 
please let us know the location, level, mobs, players, etc so that we 
can reproduce the issue and research it. Remember though that higher 
fire/frost/etc resistances can allow mobs/players to resist more.

Note: Since patch 1.7, Feign Death can no longer be resisted by players.