SN has slow BBs which turn worse than other nations, and high angle guns with huge hang time, but the second best range, high damage and moderate reload. The spread is inconsistent for most guns, you can get a tight shot one salvo then a spread one the next, but the spread is never really that bad. They have decent, usable AA. They're moderately durable too, and best suited for line fighting. They actually have some of the strongest armor, but no where near enough free space to fit a worthwhile amount beyond the standard 0.2" belt and bulge. They also have very good engineers, so although you won't be going very fast, you can OH for a very long time.
MN has fast BBs which do very high damage with guns normally 35-40 degrees, but with range around the shortest of each tier. Their reload and spread are both pretty average. Their ships are also quite soft, although naturally high restore means they can cap SD slightly earlier than others. They have excellent long range AA, but they need it because they cant line fight and rely on opportunity strikes from support/rushing/counter rushing. Most of their ships dont have aft guns, so they can't play dodging and slinging as well as others can. Not sure how good their armor is, considering their reliance on speed I doubt its worth using, but you do see the odd MN BB5 (the Alsace) which is running armor, or faking it.
RM has mostly small BBs which all run 30 degree guns with very short hang time making them pretty easy to hit with, although their accuracy is less than impressive. Their rangge is fairly high, but their damage is generally not, they rely on reload and low angles to effectively fight. They tend to turn slightly better than average and their speed is above average, but their OH time is poor. Their guns have naturally good reload speeds, although they lose this edge at high tiers, however their gunners have huge reload growth and they can cap it very early if you pile on the vets, but remember you have to struggle with probably the worst spread of any nation until higher tiers. Defensively theyre like RN, except without the high quality armor, they line fight fairly well because of their above average speed and turning, but they also sometimes hang back and wait for opportunities because although they are fast they have the shortest OH time. They have great repairers, second only to RN, and are about average in durability. Their AA is basically non-existent, possible but nor advised.
SN fighters are the slowest of all, but have very high attack and good AP defense, so they are very good when in a fight, but easy to outrun and evade. Their fighter pilots are average.
SN DBs are also the slowest, have very little fuel and uninspiring defense. They do, however, have the strongest bombs of all nations, capable of dealing more damage with DBs than anyone else. SN bomber pilots are also the best, this applies to TBs too.
SN TBs are quite poor. Although the TB aircraft have good fuel and sight, the actual torpedoes do the worst damage of any nation however they are also the fastest moving torpedoes.
MN fighters are very good, the fighter pilots are above average (theyre in the group of nations that comes second to USN, along with KM and RM), and the aircraft are mostly very balanced, without any real weakness.
MNs prefered bombers are DBs, although worse than SN and IJN, they drop very powerful bombs with aircraft that are fast, with good sight and above average defense. Their DB pilots are average.
MN TBs are unimpressive, with the lowest ability on any TB pilots and aircraft with low fuel and defense, but at least good sight and speed. Their torpedo damage and speed is above average.
RM fighters are also very good, similar to MN and KM fighters. Well rounded fighters with good values for all stats, although with an emphasis on ap defense and DP (health) although they are fairly slow.
RM DBs aren't the best. Their bomb damage is close to the low end, similar to RN the damage is not optimal. Their DB pilots are also at the low end of the spectrum for ability. But nations that tend to have worse bombs normally have better aircraft, and RMs DB has good overall stats with great sight and fuel, but again they are rather slow.
RM TBs however are very good, second best after RN, their torpedoes do the second best damage but are slow moving too. RM TB pilots aren't great, they would have the lowest ability for TB pilots if it wasn't for US TB pilots, but their aircraft have great sight, best fuel, average speed and good defense.
I'll leave SS for someone else to tell you about, but SS are currently undergoing a rebalancing process so they may turn out slightly different than advertised here before long.