Actually, it's unfair to compare this to
Kanon and
CLANNAD - not unfair to Anohana, but to Key. If you only consider the works as pure works and disregard the differences between the original games and their anime adaptations, those are really far from the best works of Key - in fact, I'm dare to say
Kanon is the most "plain" work of Key (I'm trying to avoid the term "worst"
). They just happened to get the best resources for anime adaptation, comparing to other Key works.
With that said, though, personally I wouldn't compare Anohana to Key works. Most Key works are meant to be epic: tearjerking, giving you heartache, making you feel impacted, etc. I don't see Anohana that way. To me, Anohana is like a sketch that's meant to strike a chord with its audience through an ordinary story, and it certainly stroke one with me.
Indeed. Menma's (re)appearance is just to fill in the gap of their lives. She didn't do any magic by resolving everything; in fact, she didn't really do much. She appeared, hanged around for a bit, that's all. It's up to others to get their lives moving again. That's the merit of the show to me.