I don't believe that you
knew
for sure who Keyser Soze is before the movie's ending. I especially don't believe it based on your explanation for knowing. I've seen a few such claims and they're ludicrous. What's so hard about being honest?

Agreed. An unreliable narrator wasn't something audiences were expecting or used to in 1995.
Let's be bad guys.

I think it's much more obvious today because it was very influential. You have to keep in mind that it's a 20 year old movie now so a whole generation of filmmakers have borrowed from it and been influenced by it.
I was 25 when it came out and it was considered a genuinely impressive twist, not just because of what the twist was but because it wasn't considered gimmicky. It wasn't the first time ever, but it was one of the only times that when they made the big reveal, you wouldn't think it was stupid. Like a lot of times all that happens is they suddenly make the least obvious person into the most dangerous one, with no setup at all, so you're left feeling that there's no reason for it to be this person EXCEPT that it will surprise the audience.
But in the Usual Suspects, if you're looking for it, you can see Kevin Spacey scanning the wall with his eyes and thinking deeply about his story whenever he gets a chance, and he even has a genuine reaction of being pissed and upset when the other cop barges in with some news (I can't remember what he says off the top of my head, but Verbal curses angrily or something before recovering his composure.) It's in the movie the whole time that he's Keyser Soze, so it rewards multiple viewings, or attentive first viewings.
So it wasn't just the fact of the twist, although that wasn't as played out 20 years ago as it is now, it was also the skill with which it was executed from the point of view of the directing and acting.