Tunnel vision is the disregard for everything but your central task, an intense focus on the one thing (and only that thing) you are engaged in. I see tunnel vision hit healers mostly, and I blame the fact that they typically target raid members, and not the boss. As a tank, I have the boss targetted at all times. I have a much more accurate feel for the boss fight because I can watch his health progress. DPS are the same, but since healers don't usually have the boss' health in view, I feel like they can easily lose track of the fight's progress.
Last night, it was a mess of cooldowns being blown at the absolute worst times. Bloodlust at 3%? Really? Our healers were hurting because they had their faces buried in the tank frames, healing their rears off. Normally that's a good thing, a great thing for them to be so focused on tank healing, but in general, I don't think there's any real need for it.
Let me start off by first contradicting myself. There are fights where extreme tunnel vision is allowed and somewhat encouraged. Brutallus comes to mind. Pallies could not stop casting or the tank would die. One missed heal, the tank would die. For the healers, nothing else mattered besides keeping the tank up, so tunnel vision was ok.
And now the bad. In fights like Patchwerk, which I slightly compare to Brutallus, tanks are the only ones taking any damage, so tunnel vision on that tank is a good thing, right? I say wrong. Heroic Naxx involves multiple tanks taking Strikes, and while you as a healer may only have one tank target to heal, you need to be aware of your surroundings in order to best help the raid. Completely non-healing issues such as the aforementioned Bloodlust example come to mind. I realize, healers, that your job is to keep the tanks alive, but the rest of the raid's job is to kill the boss. I say the same for you that I do for DPS who insist on not having raid frames (They reason because they don't care about the raid's health, they're just here to kill the boss): tanks, healers, and DPS all interact and meld to kill the boss. The tank's job matters to the healers and DPS, the healer's job matters to the tank and DPS, and the DPS's job matters to the healers and tanks. I think we need to start a WoW Environmental Awareness campaign, because this is the second issue I've dealt with now that directly involve a need for greater spacial awareness while playing the game.