it's a sequel, don't expect it to change too much to the original formula.. it's not like Halo 2 completely changed halo 1.. I'm personally guessing more weapons, more maps and maybe some more modes too... though ofcourse i'm completely open to the idea of the game having a whole bunch of unshowcased innovations and i do think it has at least a few things we haven't been showed yet.
The thing is, Valve aren't like other developers. When they make a full sequel, they normally make it hugely more advanced than the previous, and they normally dont release something unless they think that it is as big of a jump that the previous game made. The reason they can do this is because they are an independant developer who has no pressure from non-gaming forces like Bungie got from Microsoft. This is also the reason they take so long to make even the smallest of games. New weapons, maps and modes are not enough since Team Fortress 2 got that for free. Even with the Director upgrades, it's not enough for a full Valve sequel. I would expect this from a company like Bungie, but Valve have always been different ever since their first ever game. They take risks and do everything they can to get the best game and the best value to their customers.
I really hope that they are sticking to their principles of customer service and great game design, or else I will be sorely disappointed. If it does end up as a worst case scenario, Valve will learn from it, just like they did with HL2 Ep 1.
though yes, i agree that they could've waited a bit longer with the creation of a entirely new left 4 dead game and maybe release some expansions for the first one first
They dont make expansions, especially not for an online game like L4D. They make updates, like they did with TF2. It's generally the same amount of content, but it comes out gradually and is free. It's what Steam was originally made for. delivering updates without forcing people to search it out and download before they can play again.