From Diablo Wiki
Chat channels were first enabled in the Diablo III beta in patch 13 on February 18, 2012, when they received a largely negative fan reaction due to the tiny window size and the general lack of communication-friendly features. The system was actually robust in function, but it seemed to have been designed purely as a way for quick chat and game creation; not as a system for socializing or online communication, which is what most fans were hopin for.
Aside from Battle.net chat channels, players can use their Battle.net Friends List for communication, but only with people already on their list; it's not a tool to meet new people online. Official and fan forums are also useful tools for advertising item sales or meeting new friends to play with, and there are unofficial means of live chat, such as the live IRC chat run by fansite Diablo.IncGamers.com.
Chat Channels for Diablo III
Blizzard intended to phase chat channels out of B.net with their newer games, starting with Starcraft 2. They did not include chat channels in the design, and resisted adding them until continuous player requests wore them down.
Chat channels were finally implemented into Starcraft 2 in January 2011, 7 months after the game's release. After the long wait, their function proved highly-unpopular with players, due to a lack of essential chat channel functions such as public chat rooms, private channels with mod support, guild support in chat, and more.
With that precedent, Diablo players were apprehensive, yet still hopeful for good chat support in Diablo III. The fact that chat channels would exist in Diablo III was first confirmed by Bashiok in October 2010, while the feature was under construction for the recently-released Starcraft 2.
As mentioned above, chat channels with a minimal feature set debuted in Starcraft 2 in January 2011. Fans accepted their initial form, expecting improvements over time. The fact that no such improvements were made during the next year is what brought fan dissatisfaction to a boil.
The next news about Diablo III's chat channels came in September, 2011, via a Bashiok forum post.
Immediate fan pushback spurred Bashiok to further explain why the Diablo III developers felt that public chat channels were a bad idea.
This policy decision prompted massive fan outcry, with fifty-page comment threads sprouting on Battle.net and perplexed editorials on fansites. After several days of this, Bashiok announced that there were public chat channels after all, and that he'd been in error when he said they had been removed from the game. 
While most fans were happy about this news, quite a few people suspected that Bashiok was being ordered to fall on his sword to cover for a policy change, and that his first comment, and later explanation, had been 100% correct that Blizzard was not planning to include public chat in Diablo III, just as they do not in Starcraft 2.
Whatever the case may be on that controversy, Diablo III's chat channels debuted in the Beta Patch 13 to general fan dismay. The main source of discontent was the small size and lacking appearance controls.
Despite the negative fan reaction, the developers have said nothing about any plans to update or improve the Diablo III chat interface. Given that they've made no improvements to the much-reviled StarCraft 2 chat system in more than a year since it debuted, it's quite possible that the D3 chat will remain as it is through release, and perhaps long after.
Diablo III Chat Channel Screenshots
Sample images of the chat channel function, showing off the features and options, can be seen below.
There's even a short video that documents the D3 chat channel features.
Past Blizzard Game Chat Channels
All of Blizzard's Battle.net titles have featured some form of chat channel feature. Screenshots of the various interfaces can be seen below. All are taken from this post on Diablo.IncGamers.com.
Better Communication than Diablo 2
During the game's development, the D3 Team often expressed their desire for better matchmaking and game-finding options than were present in Diablo II. The Friends List and various quest-grouping game creation options take steps towards that goal, though the systems remain under development during the beta test.
- See the Multiplayer article for more details about game creation and player interaction via Battle.net
The developers want more and easier mutiplayer gaming in Diablo 3 and they have repeatedly mentioned their desire to create quick and easy (even automated) games that group like-level characters. Diablo III Game Director Jay Wilson spoke on this from Blizzcon 2009. 
Jay Wilson: We're not quite at that stage. [Talking about B.net D3 features.] Some of our big hooks are really good matchmaking, there was some poor matchmaking in Diablo 2, it was kind of esoteric and hard to use. We want a platform that puts you right next to someone else who's into the same thing as you, at the same level. We want you to be able to open up that friends list and say, oh, my friend Steve's killing Diablo, join! And you're right there next to them. We want players to be able to play together very, very easily. That's going to be a lot of work.
It's not in the game, so this is not a promise, but I can tell you that my desire for that is that you just be able to hit Escape, menu comes up, and you can say, I want to redo what we just did. I want to replay this part of the game, restart my game essentially, with all the same people. And I just get a confirmation thing, hey, you want to? If they say yes, we go, and if they say no, they stay in that same game.Somewhere a programmer's heart exploded, but that's our intent, that you shouldn't have to have this really convoluted way to replay content. One of the other things that we'd really like to focus on is, that the endgame not actually force you…well, it doesn't force you, it's your choice, but not make the most advantageous way to play be to do the same thing over and over again.
That sort of easy partying system was in the game by the start of the beta, and all the players in a game can easily create a new one together. There are also mechanisms to allow anyone on your Friends List to instantly join your game, and ways to chat to a Friend who is not in the game with you, or even to Friends who are playing other games over Battle.net.
As of the beta there are not any improvements or customizations to the Friends List, in terms of guild support, though Blizzard remains committed to providing some such options at some point. Bashiok elaborated on this point in a forum post from September 2011.