The commission was a pillar of mayor Jim Watson's campaign, among the first announcements he made and likely the most significant promise he made for transit in the city. The composition of the commission will be somewhat larger than what Watson had originally suggested, with 12 members total: Eight councillors, and four members of the public with some specialization in transit issues (Watson's original plan was "probably 5-6 members of council, and probably 2-3 members of the public").
The commission wasn't universally supported by council, and some other commenters--notably David Reevely--have publicly wondered what it will do that the transit committee couldn't. The committee, in Watson's vision, will be able to focus more on the operation of the transit utility, and less on political motivations. The establishment of some arm's-length committee was also among the key recommendations of Larry O'Brien's mayor's task force on transportation, and both O'Brien and Andy Haydon were two other mayoral candidates who had announced their intention to establish a transit commission, if elected mayor.
Specifics aren't clear at the moment, so it's unclear what the qualifications are for a member of the public to apply for consideration on the committee, nor what the process would be. I will offer that information when it becomes available.