I have not seen the curse mark on Kabuto. (I have watched till episode 120). Why didn't Orochimaru give the curse mark to Kabuto? Is it because Kabuto is already powerful?
Let's begin by analysing people who Orochimaru marked with Cursed Mark and what they have in common.
Jiroubo, Ukon, Sakon, Tayuya, and Kidoumaru
They are granted the Cursed Mark not because Orochimaru wanted to use them as their vessel, but rather as an experiment on how far the cursed mark can improve someone's fighting capability and what are the side effects. They are also granted the Cursed Mark as a mean of improving their fighting capability so that they can serve Orochimaru better.
He was supposed to be the container for Orochimaru. He was chosen for he has the Shikotsumyaku Kekkei Genkai which enables him to use his bone as weapon. He was the last one of his clan, which makes his Kekkei Genkai unique. However, since he was sickly, he lost his position as the next container for Orochimaru to Uchiha Sasuke who possessed the Kekkei Genkai Sharingan. While Sharingan was not as rare as Kimimaro's Kekkei Genkai, Uchiha Sasuke was in good health. Kimimaro was barely hanging alive, having to rely on machineries to keep himself alive as long as possible. The thing is, the dojutsu that Orochimaru used to transfer his soul to another body (Furou Fushi no Jutsu or Living Corpse Reincarnation in English) has a limitation of 3 years cooldown before it can be used again. If during those 3 years the vessel dies out, he won't be able to escape death. Thus, Kimimaro lost his position as the next vessel to Sasuke.
Originally, it was not Sasuke that Orochimaru wanted. It was Uchiha Itachi, Sasuke's elder brother. However, since Itachi is far too strong for Orochimaru to handle (Orochimaru did tried to attack Itachi, only to fall to the latter's genjutsu and was unable to break free at all), Sasuke was chosen.
Now, why is it that Yakushi Kabuto didn't get any Cursed Mark?
First, at that time, Kabuto was Orochimaru's assistant. He is necessary for Orochimaru to help with his research, and to help him prepare the next vessel for his Incarnation jutsu. Orochimaru needed his brain more than his body. Of course if Orochimaru was in a dire need of a new vessel right away and no one else was around for him to transfer to, undoubtedly, he would have consumed Kabuto for the sake of his own survival and Kabuto would definitely give his body gladly, seeing how devoted he is to Orochimaru. However, such a condition was never the case for Orochimaru, and thus there is no need for him to give Kabuto a Cursed Mark since Cursed Mark only increase fighting ability and not intelligence.
Second, while Kabuto were said to be around Kakashi's level (Kakashi is in Special Jounin level and the fact that he was nominated to be a Hokage means that he was one of the strongest in Konoha), Kabuto possessed no Kekkei Genkai. In other words, he was not a potential vessel as far as Orochimaru is concerned.
Third, based on the trivia, the seal that was given to the Sound Four was based on cardinal direction. Since all 4 of the basic cardinal directions was filled, and that Heaven and Earth seal was used on Sasuke and Kimimaro, there is no other seal for Orochimaru to use on Kabuto.
Because Kabuto was not his experiment but a partner. It can be clearly seen that he showed affection to none of his experiments and even Sasuke was suppose to be his next body. But on the other hand Kabuto was his partner, friend or companion. Even it can be argue that Orochimaru revival to the good side was due to the action of Kabuto. So he didn't give him the cursed mark because he never wanted to use him as a pawn as much as he done with others. He might even sympathies with Kabuto for his past from childhood shown in Anime.