When people say that the lack of action wasn't the only problem, it's accurate because what we were left with just wasn't that engaging. More action would only, at best, have helped distract us from it, not enhance it like it did with other movies. The biggest problem with the movie was with what it had, not what it didn't. There are many ways to make a more mature/thoughtful/etc. Superman story without having to dive feet-first into somber and forlorn soap opera territory. I believe that Singer felt that audiences would have an inherent reverence for the character (and the Donner movie) that would somehow make that sullen melodrama more interesting by the simple fact that it was Superman....but it didn't take. It was a ponderous and uninteresting story no matter who it was about....and with Supes in particular, it eviscerates a lot of what people who have any familiarity with Superman want to see. Quite an astonishing achievement, really, in a morbid sort of way.
And to top it off, he didn't even deal with it in a heroic or upstanding way. In fact, the only one who did was Richard....and he ends up being the guy who stands to lose the most after everything's said and done! Did Supes learn something through being powerless in such personal issues that made him a better Superman for going through it? No...the only revelation was a biological one...accidental, to boot.....whose consequences could end up being a lot more damaging to others...never mind his happy surprise.. Sure, his power/speed/friendliness can't solve everything....but neither can anything else he's got, really, in this case. And it's his doing. Way to go, Superman.
So overall, if you're not going to have some serious and engaging action in a movie like this, then you better make what is there fascinating, or at least interesting enough that the character's presence adds an interesting perspective to it, and make it its own story/continuity....especially if you want to claim a popular contemporary place in this environment/genre. SR underperformed in that area quite considerably....and unfortunately, it had nothing else to offer outside of it to at least offer a relief. It could have been an effective comedy if they actually recognized and worked off of the irony, but as it stands, it's more of a tragedy with no lesson.
That said, it did still (somehow) gross $391M, so the Superman name itself has got to still be worth something. But if they want to get more than that with that kind of investment, and hopefully attract the kind of moviegoer enthusiasm to continue on, they'll need more than 'action' to help it out. And it's pretty clear that they understand and accept that....hence rebooting and dropping the melodramatic story elements that bogged the last film down, but would invariably have to be readdressed/resolved if doing a sequel.