Sep 2nd 2009 6:09PM "Overuse of Powerword:Shield: Shields are very useful. However, overusing them is bad because it is mana-inefficient. Newbie healers heavily use shields in solo play, so priests who spam them are often new to instance healing."
Now that Discipline is a useful PVE healing spec, this guide is a bit out of date. The rest of the page gave pretty solid advice, though!