With the coming of allies rules in 6th edition, I'm looking forward to simple pleasures like mixing Chaos Marines and Guard, Guard and Daemons, Chaos Marines and Daemons, and Guard and Marines.
But there are certainly many more ways to use them; many more possible combinations. But what is really interesting me at the moment are the possibilities for counts-as, whether simple or deep. Especially for the armies that are Brothers in Arms and effectively function like one army, there are a lot of possibilities.
Wish that your Space Wolves could have a Stormtalon? Now you can, and may as well paint it up in SW colors. Upset that they don't get to use tanks named after their own primarch? Now they can. How about some Imperial Fists with special Terminator Devastator squads (Tau Broadsides)? True, you'll need some tag-along guys as well, but there will be some interesting options.
More intriguing for me are the kinds of full counts as armies we may start seeing in 6th edition given the possible combinations. I know that personally, the decision for me on whether to do my Eldar Corsairs as Craftworld Eldar or Dark Eldar has suddenly become a whole lot easier! But what about a Guard army with a strong abhuman contingent (made using Ork rules but modeled as squats/beastmen/etc.), and there's even more potential for lots of AdMech armies and other things I can't even imagine at this point.
What are you thinking of doing with allies?