Sparring is by far the most important aspect as long as you keep it at that level - i.e. working to help your partner on technique under controlled conditions. If you are looking to develop power and stamina, then the thai pads and heavy bags are for you. Seeker is exactly correct in what he said about being controlled and light.
A lot of guys and gyms don't understand the difference between sparring and fighting, especially beginners who don't have control or are not relaxed or are just trying to prove something.
Unfortunately, a lot of people never aspire to fight or even spar because they believe and have experienced the mindset that every night they go to a gym that they have to 'fight'. Nobody can last with that kind of training regimen.