Old Snake - yes, it's still Solid Snake, only his codename (or at least that's what we think it is) has been changed. His appearance though is of an old man, advanced cellular degeneration caused by the cloning process used to create him has aged him prematurely. Having said that though, Snake has been giving a muscle suit to ensure that his movements in-game resemble those of previous incarnations. This is, more than likely, the last canonical game to feature Solid Snake, as the trailers have revealed that he has 3 months to live. He'll be on a mission to stop Liquid from launching an armed insurrection against the Patriots and the world.
Liquid Ocelot - now the entity comprising of Revolver Ocelot and Liquid Snake. Liquid has gained control of the 5 largest PMC's (Private Military Corporations) and plans to use them to launch an attack on the U.S. and the Patriots. Just how much Liquid or Ocelot is in control of the new entity is unknown, and will certainly be a prominent part of the plot.
Otacon - still Snake's intrepid ally, it's expected that he will have a much more hands-on role in the game this time. He has been taking care of Olga's daughter Sunny - who was mentioned in MGS2. Whether Otacon will be killed or not remains to be seen...
Raiden - the most controversial character in Metal Gear history is back, and by the looks of it he's set to earn the love and respect his creator hoped for afterall. All we know is that Raiden's body is falling apart, likely from the nanomachines placed in his body by the Patriots in MGS2, and now all that remains is his head and spine, encased in a Cyborg Ninja suit.
Colonel Campbell - returns working for a UN Security Council advisory body that monitors PMC activity, hence the interest in Liquid Ocelot and his five PMC's. It's understood that it is he who enlists Snake to carry out the mission to terminate Liquid.
Meryl - now in command of Rat Patrol Team 01, "the new Foxhound" as she calls it in one of the trailers. It's understood that she is there to monitor Liquid's activities and then take him by force.
The Patriots - strictly not a "character" per say, but as the central theme to the game they deserve a place here. Ever since MGS2 fans have wanted to know who and what are the Patriots. Other than the fact that we know they control how everything plays out in the world and that their origins began in MGS3, we know very little else. The closing lines in MGS2 which revealed they had all been dead for a hundred years only upped the expectation for the conclusion of this part of the story. The current theory now is that the Patriots have somehow managed to "digitize" life....
Mei Ling, Rose, Naomi Hunter - all return since their last appearence in MGS1. While Mei Ling and Rose are appearing as (from what we know) minor characters, Naomi is returning as a central character, as she is the only one who knows how FOXDIE and the nanomachine technology works, ie the control Liquid wants to launch his attack. Trailers suggest that she has already been captured by Liquid prior to the game's beginning.
Drebin - a new character who players will use to buy and sell weapons and upgrades. Little is known about him other than the fact that he can unlock ID weapons and make them available to you. His monkey sidekick is also interesting aswell as the nasty scar on the side of his head.
Vamp - "the blood sucking freak" is back. It is believed he is part of Liquid's private army, as he wears similar gear. All we have seen so far is him fighting Raiden and then a brief image of him glaring upwards with a huge bullet wound in the head.
EVA - or better known as "Big Mamma" in this game. Back from MGS3, now an old woman, EVA's role is as yet unknown, but trailers have suggested that she teams up with Snake and is therefore on the good side, fighting Liquid.
By the way, those screenshots above are all in-game. Yes, the game does look that good.