I think it's much too early to write a full critique of the game; I'll save that for later. But right now, I'd like to write down some of my thoughts about each class.
- Scout: Scouts have an enormous speed advantage over all the other classes, able to outrun even sentry gun flak. Double-jumping is extremely useful and really sets the Scout apart as a choice offensive runner. The baseball bat is just as fun to use as it looks.
- Soldier: Soldiers are a lot easier to play in TF2 than in TFC, because handling the new rocket launcher feels a lot more intuitive. I haven't yet mastered rocket-jumping, but I have a feeling it might unlock more potential for this class. Currently, the Soldier's offensive capability is outmatched by the Demoman.
- Pyro: Although extremely limited at long ranges, the Pyro is ridiculously powerful in close quarters, just as promised. The flamethrower wreaks havoc in crowded corridors, and it can often decide the fate of skirmishes into enemy bases. Keep a Pyro at every choke point, and watch your back for jets of fire when you turn corners.
- Demoman: Somewhat overpowered. And currently my favorite class to play. Two weapons with explosive ammunition and versatile range? Sticky bombs that cling to walls? A bottle of whisky with which to club people? What were they thinking? Nevermind - I like the way they think.
- Heavy: The Heavy has been nerfed a bit, now that he lacks an armor gauge in addition to his large amount of health. Still very useful on defense and a great counter to the Pyro or Demoman. However, the Heavy really shines in conjunction with a Medic. Offensively, this duo is nearly unstoppable, and it's the best choice for securing vulnerable capture points.
- Engineer: Dispensers are now a much greater asset at critical defense points, serving the role of both medic and resupply. Sentry guns are more vulnerable than before, but a smart engineer can maintain them without much trouble. Players are still puzzling out the best locations for teleporter entrances/exits - as in TFC, they will probably have an enormous impact on map-specific strategies.
- Medic: Probably the most necessary support class for any successful team strategy. The Medic has been completely reinvented for the better - the Medi-Gun is reminiscent of the Recovery Pack from Tribes 2, except that it's much more powerful and easier to use. Even against overwhelming force, a single Medic can completely turn the odds in his team's favor.
- Sniper: The sniper rifle no longer uses a click-release method for building up power. Instead, your shot becomes more damaging the longer you remain zoomed in. This feels a lot more realistic, and it balances the class a great deal. The Sniper is still the most effective class at long ranges, although it probably also takes the most skill to play.
- Spy: Finally, they've done justice to this class. With the ability to disguise not only his appearance but his very name, plus the ability to cloak for short durations, the Spy is the envy of chameleons and secret agents everywhere. Ideal for disabling sentry guns (with the Electro-Sapper) or taking out key players at key moments with an instantly-lethal backstab (Medics, watch your back).
I'm sure my opinion of each class will change as I put in more hours of gameplay. I've yet to play all six maps, but I'm looking forward to learning them backwards and forwards. Who knows - I might even build some maps of my own.