I think it's important that the couple shares similar views on
the basics / foundation like values (w.r.t. family, parents, money,
job ambitions), and perhaps religion. Otherwise, it could lead to a
lot of problems later on.
Rather than being similar in personalities/characteristics which I
think is boring, it's nice for each person to have complementary
characteristics which will add to the depth of the relationship and
offer a different perspective on things.
Don't forget too, that a couple does grow and change and one person
may influence the other (for better or for worse 
). E.g. in my years of
knowing SO and even after we're married, I've hardly seen him read
a book, coz I think he was lazy. Then one day, I passed him a book
which I thought he absolutely must read, and that kind of
kick-start his quest to read a lot more "intellectual" books,
whereas I just read cookbooks and books with lighter, fluffy
contents. 
I've also influenced him in other areas like in food preferences,
interests, etc. but this is just an example.
