Insomniac listened to its fans' complaints about R1, fixed them, improved everything, and then some!
Graphics - Nowadays people get very picky on graphics. If you are someone who must play a top notch graphics game, don't get R2. It has very good graphics, but nothing revolutionary. The colors are brighter and more vivid, no more of the washed up yellow-ey color from R1. Taking into consideration the amount of action going on in the game, SOMETHING had to compensate for it. Don't get me wrong, this game looks great, but its not anything new.
Audio - Solid, not bad or great. In campaign, suspenseful music is used in the right situations such as a dark room or such. In a big battle, you can hear screams of other army men, gunfire, grunts of chimera dying, explosions. It's better than R1 though.
Gameplay - Resistance is known for its run n gun type gameplay, and it works great in R2. Contrary to R1, you can only carry to guns at a time, and there aren't any guns sitting on the map online waiting to be taken. Some may like this, some may not. I honestly don't mind it because I would probably use only 2 guns in my collection of guns in R1. Gunfire battles in R1 are great, its not like other games where you shoot, charge, melee your opponent, and see who wins. Dying isn't as slow as R1, but it isn't as fast as CoD4. Its just right in the middle and it works.
Campaign - The campaign is broken up into a variety of missions, which is broken up into objectives. However, there isn't much of a story, other than Hale's resistance to the chimeran virus. The campaign is a huge step up from R1 though with a lot more action in it. New chimera as well don't hurt. People have different opinions on the ending; personally I liked it.
Multiplayer - Best aspect of Resistance. 8 player co-op is INTENSE! Teamwork is key and the spread of the 3 classes should be wide. It isnt a walk in the park either. Medics, Spec Ops, and Soldiers all need to work together to ensure a complete mission. Competitive multiplayer is great too. 30 vs 30 matches are craaazy. The maps are big enough too, its not all jumbled. Some people complain that bigger isn't necessarily better in shooters, well then, you can create a 3 vs 3 match. Berserks are a nice addition to R2. When you get a number of kills, you have your berserk and can active it whenever. Each one in unique. They range from turning invisible, to equipping a rocket launcher, to your player exploding when close to someone (no death will be penalized to you). It releases another dimension to multiplayer.
Replayability - This game makes you want to beat the campaign again. There are trophies to be collected and intel to be found. In multiplayer, you can level up your character in competitive mode to unlock more berserks and custom equipment. And in co-op, you can level up your class (medic, soldier, spec ops) to make it stronger and more secondary weapons to equip. As well as berserks in co-op mode.
Overall, R2 is a great game. No complaints here! I will be playing this for a long time.