Remember German reunification? Korean unification would look similar, only much worse. While it's not as drastic as a theoretical unification now, there was a pretty huge economic and infrastructural gap between the Koreas, like with East and West Germany. However, in the 1990s and now, North Korea was much more populous than East Germany while South Korea was less populous than West Germany, making economic and social integration much more difficult than in Germany.
But, overcoming that, the united Korean economy would be much stronger by the present day, probably becoming one of the top 8 world economies, if the north can successfully be absorbed and its mineral wealth and manpower can be optimized with modern technology. Total population would exceed 80 million (currently 75 million) at least as the famines that have devastated the north since 1990 would be averted and all the refugees fleeing into China to get to somewhere else (sometimes China, sometimes South Korea, always somewhere outside of North Korea) wouldn't be going into China and probably would stay put. So total population somewhere between 80 and 90 million, probably.
The northern border with China would become quite a bit more militarized, seeing as China's not too comfortable with the US on its border and the territorial disputes that might flare up due to Mt. Baekdu and the surrounding area, like the Yanbian province and the rest of Jiandao. While not the DMZ, it would be pretty hard to pass through from Manchuria to northern Korea.
On the DMZ though, a fun idea would be the conversion of the DMZ to a state park. Untouched for decades (due to all the mines), it'd one of the most pristine wildlife sanctuaries in the world and a potential tourist hotspot (once they get rid of all the mines).
Without a perpetually hostile neighbor, though, there wouldn't need to be a draft and the Korean military would be a great deal smaller than OTL North+South Korea's militaries put together (though that's not saying too much). Still, there'd be a strong military presence with China and Russia so close by.
Culturally, K-Dramas would have way more focus on Korean (Joseon) history back before the partition and family reunions (say a family separated during the Korean War finding their way back to each other) to help smooth over regional differences and breed a sense of fraternity. Northerners would still probably be second class citizens, though, and looked down upon as uneducated, shorter, and more prone to crime (the last bit due to being discriminated against) like Joseonjok are in OTL South Korea.
Politically, there'd be plenty of Red-Scare sorts in the south (with 2/3 of the population), so conservative parties reigning supreme would be likely for a while. Anti-Kim-ifying the region would be necessary to long term stability so the peninsula will be pretty unstable for a while. The north will likely stay as one bloc, the south might splinter later, but northern politicians will be fighting the stigma of communist sympathies for decades.
Anyways, I imagine it'd be something like the above. A mixed bag, certainly, but almost certainly better than what we have in OTL.