It's really unfortunate how rude people can be in groups sometimes. I admittedly get cranky if someone is clearly trying to cause problems, or not listening to advice, but I try to give people the benefit of the doubt, and it it seems like they're new to the game, I'd rather try to be helpful than overly critical.
You get it even when you're not new to the game though. I've been healing some low-level instances on my druid, and I don't know if they changed something dramatically or what, but I am having a seriously difficult time with it (and noticing other low-level healers having the same issues) despite being great at healing on my priest back in the day. And so I've had people yelling at me, or another healer, "You suck noob!" or "Fail healer is fail!" or whatever. I generally just ignore them and eventually they drop group while the more patient people remain.
And yeah I've actually been DPSing with Disc on my priest. I tried shadow once and wasn't a fan, but I've found disc works pretty well for questing/solo leveling as an alternative. The bursty style of casting reminds me of my mage.