One of the first things you will notice is the aiming and general control, even with KZ2 set to 100% on both x and y axis ( similar to KZ3 set at 40% ) it will feel slower but after a few games you will get used to it. Make sure in the options - controls that you have the HP ( high precision ) box ticked this removes the deadzone on the right analogue stick.
The next will be the class system in KZ2 you have to start off with the grunt with only 64 bullets and 1 grenade and you unlock the other classes one after the other you do not get to choose like you did in KZ3 ( this is far better as it forces the player to use and get experience with all the different soldier types )
As you progress you will unlock the medic class with its first ability the medic revival gun , by using your first abillity and reviving 5 players in a match you will recieve the revival specialist ribbon get 8 of these and you will unlock the medics second ability blood packs these can heal both you and teammates ( one difference with the medic gun in KZ2 is that you first have to select it with left on the dpad and then fire it with R1 it can also be used as a weapon against enemy players )
Progression on the other classes is the same 5 uses of primary ability unlocks secondary ability.
As for the low ammo and grenade count you can increase this by earning misssion ribbons.
Body count ribbon ( make 10% of all kills in a game ) once you have 8 of these you will me given starting ammo of 128 bullets ( I think it may be a bit more)
Good conduct ribbon ( earn atleast 20 points without earning any penalty pints ) this will increase your grenade count.
The 2 biggest changes you will see are the spawn on squad leader and spawn grenades.
To spawn on squad leader you first have to join a squad and then providing the leader is alive you can select him on the respawn screen by using the dpad. You will have to rank up a few levels before you are able to make your own squad.
To use spawn grenades you first have to unlock the tactician badge and once you do you select the grenade by using left on the dpad then throw it with R1 ( One tip you will spawn facing the opposite direction to what you were facing when thrown so if you wish to spawn facing an objective run up to it then turn around 180degrees before throwing it ) nothing worse than spawning in the wrong direction.
Hope this helps and well done on getting the best FPS the PS3 or any system has to offer. 
^^ This guy loves his kz2 lol..
So do I
One thing that you will notice is the spawn grenade. Imagine that...you no longer have to spend a bunch of time worrying about one stupid area of the map...now everything is in play. These warzone clan matches on kz3 are the dumbest thing I have ever seen...It's just a capture and hold thing in one area of the map...what a total joke...the spawn grenade is king and with time you learn the art of throwing a good, well-timed one.
No worries mate just hope you have as much fun as I do playing KZ2 ( three years old and its still my main game )
Remember to keep to "MY RANK ONLY " matches when you start off to avoid all the CIC's who will ruin it for you.
Also try and get your M4 pistol as early as you can, to get this you need to earn the sidearm ribbon by getting ten kills with the Helghan pistol Sta-18 this can be done by shooting enemies or melee with the Sta-18 equipped and you need 8 ribbons to get the medal then you will have the 2 hit kill M4 with unlimited ammo.
