A limit would be a good idea.....what about 3 special rules per army ? 
 I'm going to struggle with only 3 for the Eldar list..... 
 
 Holofields....haywire grenades....mandiblasters.....night spinners .....grav tanks....psykers... fleet of foot.......it goes on and on ! 
 Those pointy-eared dweebs have got more cheese than a cheese factory..... 
Added (14-Feb-2014, 11:19 PM)
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Quote zellak (

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 Will every Special Rule be available to all players/armies?
  
 I suppose some special rules for one army might fit another. 
 Depends on the fluff......