This is one of the first manga by Mori, it's about love in a society where class matters the most. You marry people you're supposed to marry and not someone from the lower class.
Of course, this is what this manga is all about and it's a difficult story to get right. You need to research England and understand how things worked in a different time period and Mori does all of this well.
The greatest strength however is the art, it's really strong and it's what makes Emma so good. If the art would be worse this wouldn't be worth reading at all.
The other strength is that while it is a love story it's not super serious, it's realistic but it isn't boring.