Before you know how the devs at Blizzard generally think when they design something, trying to understand a skill solely based on its tooltip can lead to funny misunderstandings on how a skill actually works. Anyone who's ever played a TCG, like Magic the Gathering, knows about the quarrels that can ensue due to people understanding tooltips differently. Only in WoW one doesn't have to wait long to find out how Blizzard meant it to work.
I remember really far back, I think this might've even been my first char ever. That too was an undead priest, although not the same as I now play as a main. Back then the tooltip of Fade said something like "Will temporarily make you fade out, making the enemy less likely to attack you". You may call me an imbecile (I dare you), but I interpreted this as working like Mind Soothe does, ergo out of combat if I wanted to. I thought it would work like a semi-stealth, meaning I could get a little closer to mobs than otherwise before they would attack me. I had to find out the hard way that Fade didn't work anything like that, and when I eventually turned level 20 and got Mind Soothe I realized there had to be a difference between the skills. Fade clearly didn't do what Mind Soothe was doing. Apparently I wasn't the only one who had trouble understanding the Fade tooltip since it has been changed and nowadays reads "Fade out, temporarily reducing all your threat for 10 sec". Hopefully newbie priests won't have to be as confused and dead as I turned out to be because of this mistake.
But I am not alone. Love told me he once encountered a rogue who used Feint in solo-pve (and it feels like I've told you about this already) because he "noticed a difference". The tooltip of Feint reads "(...) lowering your threat by a small amount, making the enemy less likely to attack you". Anyone who knows anything about WoW quickly understands what this is all about. But imagine someone completely new to the WoW-terminology? How would they understand it? This rogue had apparently understood it to mean that the enemy would be less likely to get a hit on him. Feint would in WoW-terms lower the mobs hit chance on him. And I understand the confusion. Threat really isn't anything about "likelyhoods" at all, so using this kind of wording could make you think it's about something else, like "likelyhood to attack" = "chance to hit".
When inspecting one of my guilds warrior tanks the other day I noticed that he hadn't specced into Last Stand. If you're only into doing heroics I understand if you don't feel the necessity for that talent, but this guy tanks us all the way to Lich King. I'd say Last Stand is a crucial talent if you're into endgame raid tanking and I told him so. He explained to me he didn't like it because he felt it "hard to use". I didn't understand that argument at all, at first. Just use it when you would otherwise use Shield Wall, I said. Combine it with Enraged Regen for extra awesomeness. He asked me why Last Stand would affect Enraged Regen and I explained that since Last Stand increases max hp, and Enraged is affected by max hp these two synch well together. My guildie was confused. Increases max hp? The tooltip doesn't state that at all. It says "(...) temporarily grants you 30% of your maximum health for 20 seconds, after the effect expires the health is lost". Reading it like that my guildie had understood it to work like a big pot. 15000 hp if you're around 50k, so a damn nice pot of course. But he felt that a temporary pot wouldn't be very usable, the situations in which it could be used safely too extreme, since the hp is lost afterwards. And I understand him, what you're after is really the hp -and- the increased max hp, something the tooltip fails to mention in a clear way.
There are many examples like this. Sometimes people just fail to read the tooltip properly, like all those shadowpriests out there that didn't read that Mind Sear doesn't affect the current target, and the like. But sometimes one has to admit that you really tried to understand a tooltip but it just doesn't really tell you the whole truth.
Can you think of any more examples of misleading tooltips?