1 - Not a video format, but a file container. The container is an open standard that relies on often proprietary codecs to deliver audio and video. Released originally in 1991 for the Macintosh, the underlying container hasn't changed much from its original specifications. This is essential, however - for the backwards compatibility that the platform delivers is unmatched by any other multimedia platform, except for perhaps Xiph.org's Ogg container.
Critics would argue that the platform is bloated, although no spyware or malware is distributed. A stand-alone installer is available for those that do not want iTunes, although iTunes is dependent upon the platform. Most of the proprietary codecs have been reverse-engineered, allowing playback in such players as VLC. Recently, Apple has moved away from standards such as Sorenson Video and qDesign music to insist on somewhat of a truly compatible model ; as supposed by the heavily patented MPEG-4 standard.
Quicktime directly competed with Real Network's real audio / video standard, Microsoft Windows Media, and Xiph's Ogg multimedia platform.
1- Quicktime is a container, not a video type.
2 - Quicktime sucks! It's so bloated! Yeah, if you try playing back high-res AVC on a Northwood... 3 - Major media outlets use Quicktime to fight off the invasion of the rather sucky Windows Media Video format.
4 - Quicktime is nice, but Theora is more useful for video despite its aging video portion..