I don't like to give away much, but assuming that you've watch the first, I will mention some aspects of that.
This is a continuation of the first season, the same characters (with a few new ones), same themes of first, ect. But this anime, unlike any other anime I've encountered tells what happens after the "boy gets girl" story and builds upon all of the themes to make this in my opinion a masterpiece of animation/film.
This anime is for a mature audience, in the concept of its themes, as it is about growing up, and the difficulties one faces. I doubt most immature people will be able to understands its messages besides its a "slice of life". The themes are: fate/destiny (in that the choices one makes has many possible outcomes(, love (in the true and honest sense [there are no sex scenes which is a welcome]), life after (high) school (in that friends often go away), realization of what it means to be a man, a woman, a parent, a husband, a wife, in a very modern sense; after all "clannad" is Irish gaelic for "family", and the use of the "The Big Dango Family" song, a continuation from the first series, Clannad, can have many layers of interpretation: song brings people together, Dango being a type of (Japanese) food (which resembles the balls that are the Dango Family on sticks) brings people together; have you ever noticed that at home with your family, the few times you are all together is probably at the dinner table? The largest theme is that nothing remains the same, and life is continuously changing, including ourselves, for if not, nothing would survive (the alternative world with the girl and the robot).
Of the few things I felt that were lacking in this anime, is the songs, not very diverse, and that it seemed to be trying too hard to make you cry (on top of the almost overdrawn out drama ... but that is what also makes it more real, as life doesn't happen as quickly as it does in any type of "slice of life" entertainment).
I highly recommend this anime, but first you have to watch the first part, Clannad. I'll be hard pressed to find another anime that has left an actual impression on me as this one has (I wrote a review after all). The impression that I got from this, is that death is something that we all face, as well as drama, happiness and laughter (which all occur at times in the series) and no matter what pain may come, with family (either friends, parents and children or some other form of social bonds) one can prevail. The series reminded me of the themes of J.R.R. Tolkien's 'The Lord of The Rings' and to add some quotes from that fantastic tale "Little by little, one travels far", "I will not say, do not weep, for not all tears are an evil" "It is not our part to master all the tides of the world, but to do what is in us for the succour of those years wherein we are set, uprooting the evil in the fields that we know, so that those who live after may have clean earth to till. What weather they shall have is not ours to rule".
A wonderfully drawn anime overall, with a great message, and a generally fantastic development of characters, well written story. Watch it, and you may need a tissue-box close by!
