I used slam 30-39 while leveling. It's basically a heroic strike that speeds up your swing timer if you time it right and use a slow weapon. Say you're using the ww axe that's 3.6 speed; slam will make your next swing do auto damage + a little extra as if it's a 1.5 - 3.0 speed weapon, depending on cast time interuption. So you can do this: charge, auto, slam, auto, slam, etc. You can fit rend in there whenever you got the rage and if the mob will last long enough to make the damage worth while. Just make sure to slam as soon as you autoattack swing. Don't try this if there's 2 or more enemies attacking you. You'll never get the cast off to make it worth it.
Rend is still worth using until 40 imho. It's decent damage for the amount of rage you spend. Bleeds ignore armour. Remember that heroic strike and slam pretty much negate any rage regen from your next auto so they cost you more than 15 rage and it's all to do that extra damage the tooltip shows (but slam gives you that added dps increase because of the quicker swing timer).
edit:
As for leveling builds using 2hand, here's what I recommend:
http://db.vanillagaming.org/?talent#LMhxbNboxuEzzh0x Arms PvP Server
http://db.vanillagaming.org/?talent#LMGxbAboxzVVzu0x Arms PvE Server
(you can change to axe spec. Don't feel limited to using just axes or swords. Big damage slow weapons should be the priority.)
http://db.vanillagaming.org/?talent#LMhxbIbZEzzhMxRVo Fury PvP Server
http://db.vanillagaming.org/?talent#LMhxbIbZVVzIMxoVo Fury PvE Server
Benefits leveling 2hand arms:
+ early tactical mastery
+ early imp. overpower, deep wounds, and impale
+ sweeping strikes (use with whirlwind for big aoe damage)
+ great pvp talents
Benefits of leveling 2hand fury:
+ early cruelty
+ great bonuses to crit and being crit on
+ not entirely limited to using slow weapons when using bloodthirst
+ decent pvp talents