KGun.. I agree Opera is a great browser, I've almost stopped using it since version 8 because it's slower than FF now, the initial start up takes a while and FF with addons has many handy features I love.
Generally when you do a site if it works in FF correctly it does in Opera also.. but here's one site I'm redesigning that's fine in IE and FF that messes up in Opera badly...
Golf Kelowna! I haven't found where I messed it up as yet though.. still working on that input graciously accepted.
Also many many apps (game sites are the most prevalent but there's others too) that only run in IE will also work in FF but almost never in Opera which can be frustrating having to change browsers constantly.. I love the FF addon to open page in browser (I have both the IE and Opera one) so with a quick right click I can view sites in all three...