I believe that all serious students of any martial art should take time to study their art's origins, and if possible, the intent of their art's founders. Luckily, my sensei feels the same way. He's lately been interested in (and thereby got me interested in) a comparison between the seminal works of the founders of aikido and karate. In that spirit, I thought I'd take a moment to jot down my take on the matter. In future posts, I'll tie all this in to my recent research effort on the book Spirit of Aikido by Kisshomaru Ueshiba.
In karate we see a colorful history, some points of which are more clear than others. It looks to me like a handful of Okinawan masters were responsible for introducing what we today call karate to mainland Japan, and this introduction is largely responsible for the differences in style. In aikido, it's a little bit different.
The history of aikido is, for one thing, shorter than the history of karate. Another difference is that the early development of aikido occurred entirely within Japan, by Japanese people (karate is heavily influenced by the principles of Chinese martial arts, and further developed by Okinawans as a means of defense, in part against the Japanese). One result of this difference is that aikido includes spiritual concepts from the Japanese version of Buddhism and from Shinto. Of course, the Omoto religion, to which the founder of aikido adhered (and which is infused into the heart and soul of aikido), is entirely Japanese.