Being from California I don't have a lot of experience with subways I suppose but I have used Tokyo's subways for 7 years and so when trying to use the subways in Stockholm it's obvious I'm going to compare them.
Sweden is supposedly known for design but apparently that's only visual design and not functional design because the subways are sorely lacking in functional design.
Some of the issues I ran into.
1) I walk into central station which handles both trains and subways. I try to follow the signs to the subways. I follow one which suggests subways "this way". I get to some area where I'm not sure where to go. I think it might be down the stairs but there is no sign above the stairs so there is nothing to see, no direction. There is a sign several meters before the stairs but from where I am it's not readable. Good design would put the sign such that if you are standing at the top of the stairs you can tell where the stairs will take you.
2) Subway lines are labeled by number as in take line #13. They also have a color. But several trains share the same color. For example both #13 and #14 are "red". Maybe that's okay as I guess they mostly share the same route and are only different at the ends but it's confusing. Especially because often you're told "Take line 13" but at the station the numbers for the lines are very small so it's hard to find #13. In other words you get to the station, signs are in color for the red lines, blue lines, green lines, etc. But you don't know which color you want you only know you want #13. Translating from a number to color requires finding a map of which there are few available.
3) The subway trains themselves are all the same color. In Japan, the orange line has orange trains, the purple line has purple trains, etc. In Stockholm all the trains are blue.
4) There are no signs on the platforms telling you which stations you're at nor which stops are previous and next. Compare to Japan where every station has multiple signs showing the name of the station, the direction of travel and both the previous and next stations. Example
You can clearly tell you are at Harajuku station, you came from Shibuya and you're going to Yoyogi. You can even tell you're on the green line.
4) In Stockholm there are no maps on the platforms so whether you should go one direction or the other to get to your destination is not clear. In contrast in Japan there are multiple maps. One set of maps will show a side view of the station detailing stairs, escalators, elevators and exits and which main attractions/buildings/etc are at each exit. There are also maps showing you which car you should get on depending on where you want to get off and/or where you want to transfer. For example if you want to go from Shibuya to Nakano there will be signs are Shibuya station that tell you you want to be on cars 3, 7 or 10 transfer to the Chou line (which your directions have told you you need to transfer to at Shibuya)
5) Train platforms have numbers, Trains have numbers, Subways have numbers, which leads to going to the wrong place. I was looking for #19. I see a sign for #19. Turns out that's platform #19 for trains, not subway #19.
In Japan, first off they don't give different numbers to trains on the same line that only differ routes at the ends. You could argue having different numbers is useful so you don't take the wrong one. On the other hand, someone going to the ends of the line are probably locals and can easily follow directions in Swedish where as anyone visiting, even other Swedes, would be far less confused going to more likely destinations if they were told "take the red line to ABCstan".
Then, because most Japanese trains have a different color and/or are not numbered it's easier to find the trains. Most of the time you can just follow the colors. You want the purple line there are clear signs all over telling you which way to the purple line. If it's not colored line at least they're not using 2 numbers like "train #5 on platform #6". Instead it would be "NameOfTrain on platform #6"