Where to start, at first I thought that this would be a close follow up of Ore no Imouto, but upon closer inspection this was a complete opposite. Now the reason I took up this anime was because of the fact that I rather enjoyed Ore no Imouto, while attempting to ignore Kirino's extremely obvious and annoying attempt in suppressing her emotions for her brother. I'm not saying I hate this anime really, rather it's quite enjoying, to some at least. To some I have recently found out that is extremely disturbing for reasons of their own, you know who you are.
One of the many reasons I have kept this anime on my watch list for this long is rather simple, and thats the comedy factor. Sure we've all seen animes which had a bit of ecchi or rude jokes but I have not yet seen one that has gone this far. And you have to admit it dirty and rude jokes are hilarious, no matter in what context. Onii chan no Koto has managed to satisfy that dark humorous side of ours, and even if some people don't appreciate it at one stage of their life they used to giggle like little kids when hearing a dirty word.
One of the most important factors of Onii chan no Koto is the art. I'm not sure where I have seen this art style before but although it is not the best and not that much to my taste, it does work extremely well with its humorous antics. Although not appealing as to cause an extreme uproar in the ecchi factor, which is a good thing by the way, the art certainly does make it an anime which does not fully concentrate on the desires of man; while of course moderately giving small tastes of it. I also believe that the antics the characters get up to is much more appealing to this sort of anime than pure fan service. By using the power of words and rather rude suggestions instead of pure fanservice comedy, it has effectively made it a much more amusing anime to people who actual appreciate this sort of comedy.
Of course being a male directed anime it certainly has a lot of qualities that draw people in. Music may be one of them depending on your tastes, to me ... it's just another happy go lucky tune. Generic really but so is a lot of things. Seiyuus also do a brilliant job, to me it does not matter who is the seiyuu but it matters to me how good they do their job. It could be any seiyuu who I have never heard of, but if they're extremely good and I mean this as the voice fits the character and is smooth, I wouldn't care who was acting. And the seiyuus in Onii chan no Koto definitely match this job description.
I have noticed a trend with openings and endings nowadays, which involve people running or turning their heads around, it does not matter much as all I pay attention to is the music. It's not bad in a way, extremely generic, but then again ... what isn't these days?