Even though you haven't been playing long, your assessment on those 5 civs is quite accurate.
I say quite accurate because nearly all civs can be played well and can be quite dominant in most situations. Basically, all civs are equal but some are more equal than others.
Basically, S-tier civs require fewer or zero "if" statements to be dominant. Korea is widely regarded as one of the best civs in the game because their only "if" is: "If you have hills, you're good." Unless you get incredibly unlucky or purposefully choose a map with little or no hills, Korea will be able to exploit its main bonus and exploit it hard.
On the other hand, as you noted, Norway has a very good bonus -- raiding with all melee ships starting immediately when you start building ships. But there are too many ifs that are somewhat unlikely for you to be able to actually exploit that bonus. You must -- settle on the coast; build ships; have a neighbor somewhat nearby; they must have cities and/or many tile improvements or civilians on the coast. In addition to that, there's usually not that much to raid in the early game even if those situations are met. On the plus side, you can do that with caravels in the late medieval/early renaissance so that's neat (although privateers aren't far behind)
Spain is similar in this regard except they don't have any real trait that sticks out as being incredibly dominant. They have a hodge-podge of several meh to good traits. The biggest thing holding them back is basically everything requires being on a different continent. Depending on your roll, that could be very difficult to pull-off. Sometimes it's so difficult that it's wiser to not even try because the effort in doing it won't pay back dividends.
Poland is a good example of how the opposite (inverse? converse?) of Spain can play-out. Poland is a good, but not great civ. They have no "WOW!" bonuses in particular (well, maybe the winged hussar but that's on the culture tree and Poland gets one tiny-little bonus towards culture). However, Poland is diversified enough to where you will always be using at least some of their abilities. When you become familiar with their many moving parts, you can start exploiting traits and their synergies all over the place almost regardless of your starting location.
So, the nomenclature is basically -- if you have to say "in this situation" more with a civ, it's generally considered lower-tier. If you don't have to say it at all or it's so common that it doesn't really matter, it's probably a high-tier civ. That's exactly why Rome, Japan, and Germany are solid A-tier civs. As Rome, are you building cities? If so, you're doing Rome correctly. As Japan, are you building districts? If so, you're doing Japan correctly. As Germany, are you placing two districts as soon as you settle a city and using military cards (you're forced to)? If so, you're playing Germany correctly. As Sumeria, are you building their two uniques that are available from the very beginning of the game? ... you get the point