SM for Alliance is like Deadmines for Horde, really. Also for Alliance getting to SM you don't have to go through WPL / Bulwark, it's a bit longer run but you can either head west from Southshore and go north through Tirisfal and all the mobs are L30 or below, or straight go north through Hillsbrad/Alterac and swim (waterwalk 4tw) the lake south-west of Undercity where you have some mid-high 30's mobs to avoid. I've ran several groups of L20 characters all the way there without any issues, it's a bit of a hike but no real danger.

However, thanks to the joy of having 5 accounts and therefore 50 character slots, I now have a L20 warlock with bags full of soul shards and 2 alts parked at the SM summoning stone.

Someone once did a comparison of all the zones & instances and how easy/difficult it is for Alliance and Horde to get there, they are actually about even all things considered.