If I go shadow, I have a very low HP bar and get much closer to dying (and actually die on normal quest mobs).
If I go disc (which I have for the majority) it takes me FOREVER to kill things, and I eat up mana quite quickly, typically using the shadowfiend guy every other pull or so.
Because I strictly heal (typically) I blame my lack of shadow knowledge on the dying thing, but I just level disc instead. It takes a long, long time to kill a same level mob, but at least I don't die when I accidentally get knocked back into a second (or third) one.
I said when I level my druid, I will either tank or heal, and find a leveling partner that is spec'd the opposite and ONLY quest with them.