=> 1990s setting
=> we begin and end the show with the adult versions of the female leads
=> very active female lead who loves sports
=> focuses on both the friendships and the romantic experiences of the female lead
=> two female leads shown as often chatting each other on the computer
=> most likely the audience will have a similar reaction in how the main love story in both dramas fold out
=> main difference is that there is no love triangle in 2521 while there is in 20th century girl

=> Both are Song Joong Ki dramas, both are about revenge. The people SJK is doing a revenge against are very rich/high on the social ladder (at least during the time SJK character is doing the revenge). SJK did fantastic acting in The Innocent Man, here's hoping Reborn Rich has enough where SJK can show his acting ability.

Both dramas tell the story of King Sukjong and his wives from totally different sides/perspectives. Royal Consort Choi's side is told on Dong Yi while Royal Consort Jang's side is told in Jang Ok Jung.

=> Both are dramas set in Three Kingdoms of Korea
=> Both main characters eventually became rulers
=> Both main characters were born with a bad omen, thus they had to live away from the palace in order to be not killed
=> Both main characters returned to claim their title as prince/princess when they got older.
=> Both main characters have an older sibling that loves and protects them

=> Both are long historical sageuk dramas with many episodes from the 2000s
=> Both dramas are set in Three Kingdoms Period
=> Both sageuk dramas got high ratings in Korea and was even aired in some international countries like the Philippines
=> Both main characters have at least two love interests (fans of both dramas still debate which love interest is better for the main character)
=> Both main characters have to overcome obstacles/prejudices to become a worthy ruler.
=> Both have many cool fight scenes
=> Both have actor Jung Ho bin (In Jumong, Jung Ho Bin plays Wootae, Soseono's bodyguard. In Queen Seon Deok, he played Munno, a great martial artist who is also Bidam's master)

=> Kingdom of the Winds follow Jumong's grandson
=> Song Il Gook is the male lead in both dramas (and he plays kings in both)
=> Both are stories of young men who trained and have to grow up to become a great King

Both are dramas about the founding of nation/dynasty. Both main characters in this drama eventually became kings. Epic dramas with lots of epic fight scenes. There are at least two women loving the male leads in this drama.

Both are sageuk by the same writers. Shin Se Kyung is in both dramas too.
For Arthdal fans, you might want to check 6 flying dragons for Shin Se kyung acting . 6 flying dragons was a great drama that won many awards.

Now that season 2 is confirmed with new actors, I'm sure fans of the replacements will watch season 2 for them. it is absolutely necessary you watch season 1 because it is quite a complicated world and the worldbuilding is in season 1. I also hope for some appreciation for the efforts of the season 1 cast (I'm saying this seeing how most people underestimated Arthdal season 1 drama when it was airing).
Now, if you notice the drama, it is presented in many titles. If you wonder why is that, the season 1 of the drama is divided into three parts. But all three parts comprised of season 1. This particular part here is part 2.
I also recommend season 2 for season 1 fans. Please watch the drama even with the cast change.

Now that season 2 is confirmed with new actors, I'm sure fans of the replacements will watch season 2 for them. it is absolutely necessary you watch season 1 because it is quite a complicated world and the worldbuilding is in season 1. I also hope for some appreciation for the efforts of the season 1 cast (I'm saying this seeing how most people underestimated Arthdal season 1 drama when it was airing).
Now, if you notice the drama, it is presented in many titles. If you wonder why is that, the season 1 of the drama is divided into three parts. But all three parts comprised of season 1. This particular part here is part 1.
I also recommend season 2 for season 1 fans. Please watch the drama even with the cast change.

Both are the works that show the political rivalry of Grand Prince Suyang (later King Sejo) and Kim Jong Seo.

=> Both works are about King Jeongjo
=> Han Ji Min is in both but her character in Fatal Encounter is not as nice as her character in Yi San.