I had a Priest use an Inner Fire on one of my minions (I think it was the 4/5 Yeti) just to insta-kill it. It was clever, but I still beat him.
At least from what I've seen from playing a Priest, the biggest problem is the obsession with gimmicks. Everyone has seen the gimmick once, and it's some combination of Power Word: Shield, Divine Spirit and Inner Fire to create some ridiculous minion that cries at the sight of a 2/1 Owl. The problem with this is the setup. Lightwells work so well because of their high initial health and very low mana cost. That usually allows you to get it out quickly without as much worry that the opponent will simply destroy it. Not to mention, the Lightwell can heal itself even if it takes some damage. However, it doesn't help that a lot of the current popular decks focus heavily on tons of low-cost minions (the zoo or murloc decks), which means your minion (that you're intending to buff) most likely will not be on the board alone. Some of the decks also feature a lot of low cost removal.
That makes me wonder why there isn't such a high push with Charge minions. Now, I realize that someone is going to say, "aikouka, that's because charge minions have such horrible health!" Yes, pretty much all of them do... except the 4-cost 2/5 Stormwind Knight, which actually has pretty decent health for its cost (as a comparison, the Yeti is 4/5). The only thing that worries me about that is the relatively high cost of the overall setup. The minion costs 4, Divine Spirit costs 2 and Inner Fire costs 1. So, to perform this all on the same turn, you need 7 mana. You can gain the same end result as the Lightwell, but with a +2 mana cost. I'm tempted to toss it into a deck just to try it out.
Anyway, so in my experience with the Priest, most opponents are painfully aware of what you're trying to do with your minions. Most players will try to keep your minion count as low as possible or as low of health as possible (the abilities mentioned above only work on current health not max health). My deck doesn't rely on the buff gimmick (it has them though), but I've had a lot of trouble with decks that have high amounts of low cost removal. The priest lacks a lot of AoE removal. Holy Nova deals 2 damage (and heals for 2), but it costs 5 mana. So, what happens is that the opponent uses low cost removal (Warlocks are particularly nasty with this) to kill my early minions while playing lots of cheap ones (i.e. typical zoo garbage), and I have nothing to counter that with.
EDIT:
It does seem bad at first, but when you're playing a deck that relies on spamming (Zoo Warlock or Mur-lock) low cost minions, you need to be able to keep your deck filled. When I have a really bad draw against a Warlock, it isn't uncommon that the majority of the damage dealt to the Warlock was from his own card drawing.