@Christoph142
AFAIR only the past 6 months were imported. The rest would be outdated anyway, wouldn't it?
No, it would not be outdated. Opera 15 is about 9 months old, but many users are still using Opera 12 or earlier versions. You may as well import all of the old posts. There's gold-dust in some of those old threads about customising Opera, fixing plugins, etc.
Although 90% of the threads are not worth importing, the 10% that are should not be lost to the community. Perhaps you could import all threads from the important forums — Windows, Browser, Mail, and Customising (and some others) — except those that were locked?
It is probably easier to import the whole lot rather than filtering out some.