Specifics. How is what I've described being an asshole?
In particular, as it apparently isn't immediately obvious, I'm talking about a 3 lane road. With a driver sat in the middle, you overtake them, pull ahead, then move over to their lane, then move over again to the empty inside lane (where they should be) and then slow down with them still in the middle lane. You're not brake checking them or anything, you're demonstrating more care and attention than they are, with them sat in the middle lane without overtaking and generally obstructing traffic.
What's wrong with that? It's less obtrusive than honking your horn, which arguably would be appropriate when someone's sat in the middle lane without overtaking anything.
Hah, yeah, I had to be little bit ambiguous, I couldn't just say "left, middle, right" for a 3-lane road, as that would mean different things for people from the US or Germany, and I'd probably just confuse myself trying to swap them!
Driving styles have definitely changed over the years. In the UK, they've introduced "Smart Motorways", which basically have gantries every so often with speed limit signs over them. They can very the speed limit on the fly or handle lane closures or whatever. Some also have speed cameras, but not all that many in the end and you can easily spot them - even so, people have generally slowed down to the 70mph (~120kph) speed limit now. In some respects this is good, but you also end up with cars overtaking each other very slowly with very little difference in speed, clogging up the road.
What I find really interesting is how driving styles change even across different regions of the country. You can see people behaving differently in the north and south of the UK, or even in specific areas - down near Brighton there are a lot of 2-lane A-roads where people all queue up in the outside lane waiting on 2 lorries overtaking each other, leaving the inside lane completely empty. In Europe, like I mentioned, there's this weird change when you cross the border from Germany into the Netherlands - not just the difference in speed (Netherlands seem to be very sharp on speeding, meanwhile Autobhan is generally unrestricted) but also drivers in the Netherlands pull in almost immediately, so quickly you might feel the need to slow down because they're so close in front of you.