Kim Il-sung would have remained the leader of the unified country, at least for the first few years, but his grasp on power would have been precarious. In real history, the Korean War helped him a lot. First, it led to Chinese involvement, so the Chinese could be manipulated to neutralize the Russians, who once were masters of the North.
Second, the permanent division of Korea, which is what happened in reality, also meant that domestic communists lost their power base south of the 38th Parallel, and thus could be easily slaughtered from 1953-55. And, last but not least, the war experience produced a number of people who were dedicated personal followers of Kim Il-sung and his system. The combat experience, the desire to avenge fallen comrades, and intense ideological indoctrination made many a former soldier into the "steel warriors of the Great Leader".
In our hypothetical case, things would be slightly different. The South Korean communists, the major group of internal opposition within the party leadership, would get a significant boost from such a victory, re-establishing control over their power base in the South with its far greater population. In real history, the Southerners were mercilessly slaughtered just after the war.
In our counter-factual story, Kim Il-sung would still do his best to undermine their influence, and the Russians (for a while more powerful in post-1950 Korean affairs than was the case in real history) would probably side with him - as they sided with the established regimes in Eastern Europe when the East European leaders began to hunt down and kill all their potential rivals among the communist leaders.
Still, this would be a difficult power struggle with a rather uncertain outcome. There were some real chances that Kim Il-sung himself would end up being executed as an "unmasked spy" on the US Central Intelligence Agency payroll from 1940 (never mind that there was no CIA in 1940), and a "Japanese agent in colonial times".
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But what would have been different? Would it mean that a unified Korea would have become just another version of Kim Il-sung's North? To an extent, yes. The 1960s would have been a time of frantic mobilization drives, mass brainwashing and political persecution on a grand scale. However, two things would have been different.
First, without bitter war experience, without an ample supply of the battle-hardened zealots and without daily confrontation with the rival (and also increasingly successful and free) South, the all-Korean communist regime might have been somewhat less murderous, although this might not have been the case in the 1960s, when insanely radical plans were in vogue across East Asia.
If simultaneous Chinese experience is a guide, I would suspect that those times would have added another few tens of thousands or so dead people to the regime's body count. Without the South across the border, the Pyongyang leaders would have behaved a lot more recklessly in the 1960s, as China did in the bloody decade of the Cultural Revolution. But in the course of time, liberalization would have come easier - as happened in China.
Second, without a powerful South sitting just across the border, the North would have been more willing to experiment and reform. Perhaps it would have started Chinese-style reforms at an early stage - maybe even earlier than China itself. In real history, the North has been afraid that its populace would learn too much about South Korean prosperity and that this would result in the regime's collapse. Without the South hanging around and being so provocatively prosperous and free, bolder domestic policies would have become possible.
In the long run, it is a big question whether the regime would have collapsed around 1990, or would have survived, like those of China and Vietnam. I suspect that the second option would have been more likely.
What would Seoul have looked like? Pretty much as Shenyang or Hanoi looks now (or as Seoul looked in real history back in the 1970s): crowds of cyclists on dirty streets, a few highrise buildings, an occasional slogan about the greatness of "socialism with Korean characteristics", and an occasional chauffeured car of a local cadre-turn-capitalist: light-years behind the current South Korean prosperity, light-years ahead of the current North Korean destitution.
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In a nutshell, "a great victory in the autumn of 1950" would probably have made life for the North Korean minority (one-third of the peninsular population) much more agreeable, but only at the expense of the lives of South Korean majority. The entire country would have been pretty much like Vietnam nowadays: a combination of a still poor but fast-growing economy, with an authoritarian but relatively permissive political regime.