I'd call it an anti-climax in the sense that it runs almost in direct opposition to the meaning and message of the climax at episode 18. And if this suddenly happy ending was a "reward" to the viewers then does that mean that the ending to Mai-Hime was a reward too? I'm just curious as to how this reward thing is supposed to work outside of the games context. If it's supposed to be some sort of reward as far as the anime is concerned then I'm even less than impressed.
As time goes on this whole defence of the ending is starting to get harder and harder to swallow for me. Then again maybe I'm just so used to people bitching every time a character who was thought to be dead turns up alive in some other anime I've watched (as in people just rushing straight to the boards to complain without anybody bothering to consider that they might never have died in the first place) that when another anime comes along with characters who were undeniably dead but are Deus Ex'ed so that they never died in the first place and nobody so much as blinks or goes "ara?" it just feels like a double standard in the community. A hollow sentiment and a get out of jail free card if you will.
I guess everything really is okay to most people as long as it's KeyAni that does it then, regardless of whether it hurts the overall themes of the story in the long run. No reason to get upset or anything, it's only a "reward" for the fans. Alas, as a person looking for a bit more then a reward I say nay nay.
Sorry, but if every other anime that tries this sort of ending instantly gets put in the hot seat for a little while then I don't see why Clannad should be any different just because KeyAni is so swell and everybody likes their animation and favours to the fans. I know that's become the standard as I acknowledged earlier, but I'd like to see that change and to me the same standards should apply to every anime regardless of how much one might like their production company.
Oh and like Myssa mentioned, just because the ending followed the games reward ending doesn't automatically mean that it's okay. Unless we were to assume that the games reward ending is entirely flawless, which I submit it wasn't. Of course this means any gripes I have rest entirely with Jun Maeda and not Kyoani's staff who were more or less just doing their jobs and trying not to get firebombed by die-hard Maeda fans over in Japan. In their case it's a bit of a catch-22.
Anyway, I feel that's if worth mention that if I were gauging this show solely on it's attempts to illicit emotional reactions from the viewers that it gets a flat out 10/10, but unfortunately I'm not so I have to account for a number of other areas where Clannad falls far short of the mark it set with episode 18.