Total Recommendations: 5
Both have an apathetic male lead and hyper/curious female lead. Both involve a group of unique friends who go on various adventures or solve mysteries. Both have strong artwork and soundtracks. Both also include hints of romance, drama, and comedy.
Both have shy girls who fall in love with popular boys despite obstacles. Both protagonists learn to make new friends. Both girls even have appearances that remind their classmates of other things (Sawako reminds her classmates of Sadako from The Ring, and Hiyori reminds them of a hiyoko, or baby chick).
Both series are suspensful and have a main character that is trying to obtain ultimate power to control the world or people in some way or form. In Death Note, it was through the power of the Shinigami. In Code Geass, it was through the power of Geass granted by C.C. Although one takes place in everyday life and another is supposed to be set in the future, both have a similar concept and structure.
Although the characters in Kodocha (Kodomo no Omocha) are younger than the ones in Marmalade Boy, both are older, sweet shojo series that are very easy to read and have dramatic plot twists with romance. Both have a classic shojo manga style. If you're looking for an old-fashioned, sweet, but also realistic shojo series to get hooked on, both are fairly similar in their structure, though the story lines are completely different.
Both have a young, female protagonist that uses magic to protect her friends while going to school. Both are set in a similar environment with a similar age group and positive message. Both anime are very relaxing to watch with likeable characters.