The first thing that makes it exceptional is that it breaks conventional ways anime stories are told. As Loid Forger goes about his mission, things that happen dance between realism and poetic expression not to be treated too seriously, thinking for example of the circumstances in which he proposed to his future false wife.
Then we have the fact that the story is very well written, so much so consequences of actions lead to comical situations. There's a perfect balance of detective action, comedy and slice of life.
The aspect of their family life led by people that don't know their secret identities is effectively mined for humor and plot twists, and despite all of this we are being reminded of what can be beautiful in a normal relationship and we see a bond growing between people that initially were all in it for business.
Minor flaws? The artistic freedom taken in scarce moments may be a lot to some and the final episode, although good, does not end in any sort of a bang making the fans yearning for a continuation which is inevitably coming.
But all in all reasons not to see this are: none
