The goal of the first game was to create a strong co-op game revolving around making money off contracts.
One of the important aspects in the game was the back-to-back; each guy protecting his teammate's back.
What quickly emerged were the "Walking Tanks" (highly armoured) idea and the idea of using the masks as strong identifiable icons. We liked the mask idea so much that they became the signature for our AO2 main characters.
What was hard was the fact that we were going further and further away from realism for the characters while the environments were going for a realistic look. In a realistic army game setting, it was hard to justify their look. The contrast between these human tanks and the regular people they were fighting was too different.
In the end, people really liked it and we were happy when we started to see people dressed as Salem and Rios.
I think the biggest bonus in a sequel is that you get to start from a solid technical base. The reason why (unlike movies) most game sequels are better than the originals is that you get a head start on your technology. Your team knows its boundaries and also understands the game they're making more. If we use that knowledge effectively, then we can make better decisions in the same amount of time that end up making a much better game. That basically comes from having a more experienced team that learned from the first game and knows how to work together more effectively.
When you don't have to worry about the technical issues, it allows you to focus more on the art and to push as much as you can. The big change for me was that on the first one, I was doing the concepts and the modeling on a very tight schedule. On TFD I worked with Clement Sauvé, a very talented concept artist, which had the same vision that I had of the game. He pushed the 2D, while I really enjoyed having more time for the 3D part.
The core idea of the first game was to pit a player and his or her best friend against the rest of the world, which I think still resonates very strongly. So we kept that focus and then dialled-up the quality level of everything else as high as possible. We focused on making the through-the-gun experience as powerful and exciting as possible. We added a bunch of co-op moves that the players can perform whenever they want such as mock surrender, hostage holds and pre-combat, while keeping the best co-op moves from the first game such as aggro and riot shields.
We also wanted to put players in a fresh environment. We didn't want A02 to feel like a war game, but instead feel more like a shooter trapped in a natural disaster. So we added a lot of destruction moments that feel more like earthquakes rather than explosions, even though they're triggered by the invading force. To fit with this, we wanted players to feel like they're fighting to stay alive more than fighting to beat the enemy, which I think gives the game a different feel.
Overall I think the new game is a big step up from the original, so previous fans should be very satisfied and new players should be in for a real thrill.
Visually the big change is that the first game was Axis on realism when TFD has a more "Hollywood" feeling; more entertainment, with an exaggerated reality.