From Diablo Wiki
Chat Channels were a big part of the Battle.net experience for Diablo 1 and Diablo 2. They will return to Battle.net in Diablo III, in public and private form. Chat channels were added to the Diablo III beta in patch 13 in February 2012, and received a largely negative fan reaction due to the tiny window size and the general lack of text formatting or usability features.
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 not supported during the beta, but after huge fan complaints the developers agreed to include them. They were not ready for the game's launch, but several months after Starcraft 2's release they were added in, though many fans strongly dislike their implementation.
During comments on the Starcraft 2 chat channel controversy, Bashiok said they would be included in Diablo III as well. This guarantee seemed to be in doubt in early February 2012, when Bashiok said that only private chat channels would be present, much as they are in Starcraft 2.
Immediate fan pushback spurred Bashiok to further explain why this feature was removed. 
This policy decision prompted massive fan outcry, with fifty-page comment threads sprouting on Battle.net and angry 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 at the time. Blizzard's previous efforts to force mandatory Real ID and the removal of Chat Channels from SC2, both decisions that were reversed after thousands of fan complaints, were referenced as support for the conspiracy theory.
Sample images of the chat channel function, showing off the features and options, can be seen below.
Better Communication than Diablo 2
During the game's development, the D3 Team often expressed their 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. 
- IGN: ...have you figured out anything you're planning on doing, with the exception of things like, obviously, easier ways of setting up co-op lobbies or creating panels so that everybody can immediately jump in and start up exactly where they left off?
- 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.