The biggest marketing for RO will probably the massive public beta.
A lot of people that do not like to buy a game will try out a beta. If they give beta keys to some gaming websites that gives them show time on those websites and communities resulting people playing that game and sharing their experiences.
If a lot of those people end up liking the game they will talk about it in the social networks such as forums. Especially if TWI does some good changes to the beta based on community input a lot of people can start liking it. So I guess that it will be really important if people can pick the game up and immediately "get it" and see why the game is nice.
If tripwire for instance got special editions of Heroes of Stalingrad with a RC IS2 that fires bb guns, then it might spark reviewers a bit more. Its more fun that getting a new xbox 360 bundled every day when Microsoft releases new games

.
Word of mouth has been the main method of TWI's marketing in the previous games with great success for the low to no marketing budget the game had. If twi manages to use money to actually stimulate the word of mouth advertising in a way then that probably gives the best bang for buck.