What's the difference between an "export" beer and a regular beer from the same brewer? In the UK at least, a great many beers are marketed with a regular version and a fancier (more expensive) 'export' version. For example: Carlsberg and Carlsberg 'Export'. Is there any reason why the imported beer might be reasonably different (such as the legality of certain brewing methods outside of the UK) or is this just marketing fluff?
It doesn't actually mean anything; or at least, it almost never means the same thing twice, and doesn't refer to any specific common style or process. It's about as meaningful as the fact that several dozen breweries have a beer they label as "Select".

For example, in the case of Molson Export, the story goes that it was deemed so high quality that it was "good enough to send overseas" - and, implicitly, better than competing imported brands.

One common usage is for a line with slightly higher ABV (it keeps better in shipment!) or tailored to suit the ABC laws of other jurisdictions to which it might be shipped. But there isn't any single meaning that's widely understood.