Orochimaru's main goal throughout Naruto and Naruto Shippuden was to learn all the jutsu in the world. He realized early on that their sheer number made this impossible to achieve in one lifetime, so he shifted his goal to prolonging his life the only way he knew how -- transferring his very self into another's body.
Before he encountered Sasuke, Orochimaru coveted Kimimaro's body the most. Unfortunately, Kimimaro's illness made the power transfer impossible, but his body was Orochimaru's first choice for several fascinating reasons.
Kimimaro was from the Kaguya clan, a violent and warmongering tribe. As a rule, they had no tactics in battle, preferring instead to overwhelm their enemies through sheer force and, when it applied, their large number. Each member of the clan had berserker-like strength that made up for their lack of battle strategy. More often than not, it assured their victory. Even when faced with impossible odds, the Kaguya would fight to the death rather than surrender. They were undoubtedly one of the most powerful clans introduced in all of Naruto.
The Kaguya clan also possessed a rare kekkei genkai: the Shikotsumyaku. Even among their own, the chances of it manifesting was quite low. When it awakened in Kimimaro, his fellow clan members imprisoned him until he was needed in battle. Even the Kaguya feared their own power, and for good reason.
Shikotsumyaku users were more powerful than regular members of the Kaguya clan -- as Kimimaro was the only living user, that made him the strongest in the entire clan. Kimimaro had complete control over every aspect of his bones, increasing their size or breaking down his skeletal structure as he saw fit. The ability posed no risk to him so he could use it indefinitely. He could maintain his main skeleton’s shape no matter how much he manipulated it.
Kimimaro could also create complex structures and weapons using only his bones, so he was never unarmed while the weapons he created were nigh indestructible. He was able to regulate the calcium level in his bones, making them as dense as necessary. He even once remarked that his bones were stronger than tempered steel.
But it wasn't all about offense -- Shikotsumyaku could be used defensively as well. In close combat, Kimimaro would project spikes from his skeleton at different points of his body. This strategy, apart from making him look very intimidating, prevented any opponents from attacking indiscriminately. It also increased his chances of wounding them even if he couldn’t manage a direct attack. If his opponent managed to sneak past his initial defense, he could defend with more bones at their attack site. Kimimaro left virtually no openings.
Although his spikes defended him passively, they formed the main part of his trademark taijutsu attacks, which took the form of dances. These attacks didn’t follow any set pattern or strategy, so only a very skilled ninja could escape them unscathed. Kimimaro’s ability to harden his bones also made him far more durable than the average ninja. He defended against an onslaught of Rock Lee’s furious fist and remains the only ninja to survive Gaara’s crushing Sand Coffin attack.
Orochimaru’s body transfer jutsu gave him access to all the skills and jutsu his new host had -- including special abilities like kekkei genkai. He understood just how powerful Kimimaro’s ability was and found it irresistible. Had he been able to transfer himself into Kimimaro’s body, Orochimaru would have unlocked the Shikotsumyaku as well, giving him unprecedented levels of power. It would have spelled the beginning of a dark age in the ninja world of Naruto.