> off-topic, but interesting > interesting, but off-topic

I believe you just said the same thing twice, you only reversed the order (I think the "but" here just is an AND) :-)

There's some subtlety to the arrangement of the words. "off-topic, but interesting" has the implication that 'interesting' is preferred. "interesting, but off-topic" implies that as interesting as something might be, it is still off-topic and therefore might not be permitted.

Not really. In English, "X, but Y" translates to "X, except that Y" - Y takes priority in importance over X.

It makes perfect sense when you think about it.

I did think about it. Before I posted I spent more than just a small bit of effort. I still don't see what the difference actually is though. Yes I know what "but" means. I know all those words... but...

I do not see any actual difference. Anything you apply those two descriptions to are both "off-topic" as well as "interesting", according to those descriptions. The "but" does not make any difference at all as far as I can see. The other replies say there is a preference implied (I already knew that that was the intention), but as I just wrote, however you parse it, you end up with both attributes and I don't see any preference actually being applied. I see the attempt, yes, but I don't see that there is any effect.