Sometimes I like to go play Horde because it's such a different experience than Alliance. Oh, there are similarities - there are bound to be when both factions basically level through the same zones (which they do more through the various expansions, for obvious reasons) - but the decision in the Warlords of Draenor leveling path to give Horde and Alliance their own starting zones makes a lot of sense in my mind. It reminds me of how zones like Azshara and Westfall can serve to give you a different experience leveling up, and it's that different faction experience that makes having the two factions worthwhile.
Leveling a blood elf or tauren or orc gives you a far different experience than an Alliance character (I still haven't managed to get a Forsaken past twenty, though) and it's a worthwhile one even if like me you're basically Alliance forever. Have you tried it? If you're Horde, have you ever dipped a toe over in that blue pool?