Well, I suppose it's inevitable that Futatsuki no Kishi, being a second season of Zero no Tsukaima, will be compared to the first in one way or another, especially since it's pointless to watch one without another, but I'll try to appraise its own merits as well.
First of all, as implied by the ending of the first season, FnK is a direct continuation of the story. Several new characters are introduced almost immediately, and, while I don't have anything against them, I'd like to point out that they (Agnes, particularly) take up quite an amount of screen time with their own subplots, leaving less for the overall story. However, that isn't dragged out for too long, keeping overall length of a show in mind. What comes as a good refreshment is some noticeable advance in Saito and Louise's relationship, during which several important questions are raised and decisions made. I won't elaborate to avoid spoiling.
Animation is done on a fairly high level, although it's not anything outstanding: characters are still undetailed and talk out one side of their face when shown in side view; many scenes lack smoothness — just like previously, all in all. Some scenes, particularly those of burning villages, are pretty nicely done. HD in this case is barely worth it, but on the other hand, lack of detail helps keeping the file size low (useless info for keepers).
Sound work doesn't differ from the first season at all, most themes are the same. They are fine, but still nothing really outstanding, memorable, or really touching. New opening and ending are somewhat enjoyable, definitely more so than those of the first season, IMO.
Characters… First of all, the previously known Academy residents (Kirche, Tabitha, Guiche, etc.) have much less screen time. Not sure if it's good or not; I'd certainly like to see them some more. New characters are more-or-less fine, although I can't help but think that some of them are of untraditional sexual orientation or otherwise odd. Siesta is still there, being apparently dumber than previously. Yes, everything for the sake of fan service. *sigh*
On the matter of fan service: what is particularly annoying about FnK is that the it has increased in quantity, but definitely not in quality. Saito's obsession over boobs might be funny, but it seems as if the screenplay writers put him in ecchi situations purposefully, without much concern for whether it makes sense in the story or not. You might even not notice it until the last episode, but there it will come screaming in your face regardless. Then again, the ending is rather ridiculous, anyway; seems like we'll have another bunch of weirdo newcomers in season 3. :D
All in all, with the 2nd season ZnT keeps being a good addition to your collection as long as you like harem and/or fantasy comedies, with a questionable amount of hit-and-miss fan service. Some will laugh, some might even cry, overall I think the 2nd season is almost as enjoyable as the first, barring the certain cheesiness. However, it might leave something to be desired when it comes to the story progression, so I sincerely hope they won't drag it out for too long. Any further decrease in pace coupled with further increase in fan service can easily ruin the show.