Well for one thing, North Korean purposefully refuses to use any loan words at all. This alone makes the North Korean dialect incredibly different from South Korean dialects.

I'm going to be purposely breaking the 20 year rule for this question so be forewarned if this suddenly vanishes.

Words like 'computer', 'sexy', 'chocolate', 'email', etc in South Korean are loanwords from English. Other words like odeng and 'bag' (kabang in Korean) are loanwords from Japanese.

While this doesn't sound like a huge deal, this means that its linguistically difficult to communicate many simple, everyday words to North Koreans without literal reeducation. Konglish is incredibly ubiquitous in everyday life in South Korea and it's a part of everyone's vocabulary.

I volunteer with North Koreans refugees and one of the big things that we focus on teaching them is English as well as English loanwords (Konglish). I also teach Japanese loanwords (I'm the only volunteer in Busan that speaks Japanese at my Hana center)

Usually, they are ready for life in South Korea within a year or so, which means the linguistic differences aren't so huge that NK dialects and SK dialects are unintelligible. But for North Koreans, the linguistic hurdles to significant when adapting to modern life outside of North Korea, particularly in South Korea.

That being said, North Korean dialect sounds distinctly different in pronunciation.

Also, North Korean dialects seem to say the "l/r" sound where many South Korean speakers would say 'n' or no consonant at all. Subsequently, some North Korean words are closer to the Chinese pronunciation than South Korean words, when there is a Chinese equivalent (of which there are many).

If you have more questions, feel free!