As for the weapons, I honestly just don't think the sniper rifle or the rocket launchers fit in. Killzone 2 was pretty obviously designed to be an intimate combat game, so these ranged and splash damage weapons feel out of place. Then again, with the exception of UT, I can't stand rocket launchers in any multiplayer game.
I think they would fit in if everyone had all the classes from the start. The game has two ways you can play the multiplayer: when it is that intimate combat game, kinda like CoD4, just slower with a bit more range to the combat; and then the TF4 / CoD4 hybrid where classes work together and each class has their own purpose. Right now most games are stuck in the middle, so you have over powered people running around fighting you scrub soldiers. You don't have balance among individuals and you don't have the rock, paper, scissor type of gameplay found in most class based shooters.
Something I haven't been able to parse. Are the unlockable classes stronger than the classes you start with?
In more ways than one. Even at the most basic level, there is an actual class that lets you run faster and have more health. Then there are roundabout ways of getting stronger like the ability to both repair / use manned turrets, plant a sentry and drop portable spawn points. Even the medic class, the first one you get after the original one, only differs from the default class by limiting the weapons you can use to, arguably, the best one.
but as duckman said, you are given the best all around rifle at the start.
this is the progression tree:

just for reference sake, the most points I've ever seen someone get from a single match was around 150 (so ~70 or so that gets doubled when you win.)
I'm close to unlocking the light machine guns, and after seeing that chart, I think I'm gonna slow down on how much I play multiplayer. I originally planned to rush and unlock the classes I wanted, but now I see that it won't happen even if I played everyday and all the time, at least not for a while. It is probably going to be my side multiplayer game now like L4D was a few months ago.