I noticed the other day that there are really two kinds of Tank (well actually three), the first is the tank you meet most times, generically confident and making pulls, marking up and running an instance or raid. The other type is your Main Tank type of tank, each pull they talk through it, assigning targets you should already know, warning you, watching the raid and giving position or tactical advice, and changing tactics on the fly. The third type is of course your generically bad tank that may or may not talk but generally doesn't have the confidence or experience to tank at the moment, they want others to mark the pulls and assign control.
The third type is a lot less interesting, we have all been there at a time when we had someone else help us learn an instance, unless we were at the cutting edge, which for most of us is the introduction of TBC or MgT. Thus we can dismiss them from this discussion, with experience they will become better Tanks, or just a generically bad Tank, either way they don't tend to play into raid tanking which is primarily where this difference crops up. Simply though, we can divide the other kind of Tanks into the "Main Tank", and the "Tank".
Firstly lets clarify what I mean by talking, this doesn't have to be ventrillo, team-speak or whatever other voice communication program your guild uses, it can be chat, raid warnings or emotes if needed (though thats likely confusing, did /hug mean Sheep or full out nukage). Simply its feedback to the group about what the Tank intends, their movement, the kill order and any changes to the plan. Its not to say they need to talk, a good enough group can run any instance in silence and pull it off with no mistakes, but a good Tank, and more importantly a good Raid Leader doesn't stop talking to the raid, they direct focus to the important aspects, warn you of movement etc. Its all stuff that people should do anyway but provides a central focus for the raid to work from and a central control. Of course for some fights like Archimonde its far more important for each group to talk to each other than the raid as a whole, but its a sub-set of the problem.
What difference does this make, why is talking to your raid or party important, or rather why do you have a difference, its simple, a Main Tank, or any class that is talking to the group is taking on a leadership role and making decisions, the group reacts to the Tank in advance and they are pre-warned, a non-Main-Tank simply expects the raid to react to them and do their job. This can be extrapolated to any role, and is seen in good Raid Leaders, they talk to you, you know that Magtheridon has fire patches and to move away from each other at Gruul's Shatter, but he still says it. Talking to the group focuses their attention and lets them know whats coming up in a drip feed, everyone will have read up on the fight, but hearing that a specific part is coming up makes for a better focus.
Taking this specifically to tanking, while chain pulling a suddenly moving tank can get out of range or line of sight to the healers, or as its known in the lingo, get dead. Its often said that the Tank dictates the fight and the healers move to you, but they aren't mind readers, telling them in advance lets them move in pairs or suchlike to cover you and ensure you survive. Its a level of control and experience that helps, taking Hyjal as an example, when I tank 6 abominations I am in the role of a Tank, I don't talk to the raid and expect them to heal my and overheal through knockdowns when I announce them, I expect them to do their jobs. In contrast the Main Tank in the raid (and demi-raid leader) tells the healers to overheal me, warns of high incoming damage and the type, warns the other tank to spell reflect and suchlike. I can take on the role of simply Tanking because he is providing cover for that part of the role for me, when he isn't there I take over that role and announce things, announce my plans and what is going to happen in the next while.
Its a benefit to the raid to know what is coming in from your perspective, a Tank doesn't know the DPS roles inside and out, what we know is the generics, nor can I tell you what the healing pattern on me is like, what I can tell you is the type of damage I will take, and how the fight will evolve, so I need someone to tell me if something is going badly on another part of the fight because while I might recognise it in a stress free environment, I have other concerns that are foremost in my mind.
Its something you only really notice when you look for it, normally a Raid Leader or suchlike will cover for a slacking Tank or DPS in this regard with information, but its not the same. When an Ice Nova pops, a cooldown thats important is blown or something happens, a talker will say it and perhaps what it means to keep the whole raid informed even if we were focused elsewhere. This is the difference between a good raider and a bad one, integrating with the raid, and letting people know what is going on, good or bad. We can cover for mistakes we know about, but if it slips then you need a keen eye to spot it, so tell people, whether its a macro or a yell on ventrillo, let someone know, it really does make a difference.