It is not an easy question to answer, but I will try.
Korean is a syllable-timed language while English is a stress-timed language.
As a definition of each, we can say that in syllable-timed languages,
syllables tend to follow each other at regular intervals, with an
equal amount of time being allocated for each syllable. In
stress-timed languages, on the other hand, stresses tend to occur at
regular intervals, with the result that the remaining unstressed
syllables, no matter how many in number, have to be squeezed in
between the stresses to accommodate the regular beat of the stress.
[Source: Intuition Languages.com]
When people who are accustomed to speak a stress-timed language such as English speak a syllable-timed language such as Korean, it is not easy to drop their habit and they tend to stress some syllables that are not usually stressed in Korean. It is one thing that stands out.
Also, conjugation of verbs is one of the most difficult parts in learning Korean. It usually takes a long time for many English speakers to master it perfectly as it is much more complicated than that of English.
There are many Korean words which are derived from Chinese characters. They are very difficult to understand and use unless you are familiar with Chinese characters. Some of them are difficult to pronounce, too.
The Korean language sounds flat compared with English or Chinese as it doesn't have much intonation (except for a few dialects). It is difficult for Koreans to use intonation when speaking English and it is difficult for English speakers to drop intonation when speaking Korean.