There are 3 Phases
During the Definition Phase, you can propose questions and rate them on whether or not they are a good fit for the site. During this time, users can follow the proposal.
Then there is the Commitment Phase, where users commit and refer other users to commit to the proposal which means that they will be active during the beta phase.
Finally, the Beta phase is where the proposal has a live site and is being watched for whether or not it is good enough to become an active site.
During these phases, you are only able to do the activities related in that phase. Once a proposal gets out of the definition phase, you cannot vote on questions or propose new ones. This is because the site has already been defined, and now is just looking for enough users to warrant a Beta phase.
Likewise, during the Beta phase you cannot commit to the proposal. You've either already committed, or you will be able to access the site when it goes into public beta.
This is because it would be really wrong to get 200+ people committed to a site when people are still deciding what the site is about (Questions). So naturally, once the definition phase finishes, the questions are set and you can't vote on them.
You can always look at the questions that got picked by choosing 'Definition' at the top of the proposal box. But you cannot actually act on any items, as we have moved past that phase.