*Japan: *
Have the same 203 mm guns from T5 to T9. Long reload, mediocre muzzle velocity but low drag. At low tiers they are best suited for citadel hunting, at high tiers for long range firespam. Zao gets a railgun version of these guns and they get absolutely ridiculous there.
Armour protection is mediocre.
Long range torps.
USA:
Slow but strong shells, high dpm if you get close enough. Can fire well over cover.
The best AA
Some rather durable belts (Cleveland, Baltimore, De Moines) but also the most infamous floating citadel (Pensacola). Have large superstructures that are vulnerable to HE from any range.
Long duration but short ranged radar.
*Russia: *
Long ranged firespamming railguns, on some tiers also well suited to hunt citadels.
Some very vulnerable ships (Kirov, Shchors, Chapayev).
Fast speed but poor turning.
Gains the longest range radar on T8.
*Germany: *
Good AP that has a nice combination of range, shell arcs, penetration, and rate of fire. But on T7 they have a firespammer that plays more a high tier IJN cruiser. And on the Hindenburg its just not enough anymore, Zao and Moskva have way stronger AP, so its left without any noteworthy strengths.
Good hydroacoustic search, but no radar.
Britain:
AP spam guns that can be either totally useless (at long ranges, against angled targets) or deal unmatched DPM (at short ranges, against broadsides).
Super fragile, but can heal themselves effectively.
Have smoke, which they must use effectively both to survive and to get into effective firing ranges.
Very useful torpedoes.
Gain almost DD-like characteristics at high tier, with incredible mobility and great stealth. Because they can get so close to the front lines, they make for great DD supporters or hunters.