It demonstrates complete mastery of orchestration, counterpoint, handling of musical form, and every other aspect of musical composition.
In fact, it is so good that the only comeback sourpusses have to spit at genius Mendelssohn is
"Well, he never got any better."
Mendelssohn is an example of a child prodigy who never lived up to his early promise - with the exception of the Violin Concerto. The Octet and the MSND Overture are the best things he ever wrote.
Oh and btw very little of the music that Mozart wrote as a child is all that highly regarded - apart from the curiosity of it having come from a child. He was actually a late bloomer as a composer and only started writing consistently great music in his mid 20s.