My first reaction when I read this was that it should be "ask a question."
"Start a discussion" means different things to different people - it could be that some think they will have to keep the discussion going by actively prompting for responses, etc. This might seem like a lot of work to people, especially if they are unfamiliar with the system.
One other thought - most people initially join communities/forums in order to get their own needs met, rather than to help the group (that usually comes later). "Ask a question" implies that the user will get their need met. "Start a discussion" implies that they're doing this for the good of the group.
I think it depends on the situtation. We choose to use 'Start a Discussion' and then the community member has the option to 'Mark it as a question'. Thus allowing for a conversation/discussion to happen via the first option and the latter to be 'answered'.
Also, I have been experimenting with the use of the Formatted Text widget and a link that when click opens a discussion that is pre-tagged. In this case we are using this to 1) relieve the end user of knowing how to tag or what to tag and 2) allow the community manager the option to watch the tag via their Your View tab.
We created some unique hashtags (much like Twitter) and have been able to filter information very clearly this way.
This discussion will not take place if Jive provide a "description"/purpose for its discussion forums like Sharepoint or Livelink does. The description is used to define the "scope" of the discussion item.
We define most of our forums in our COP's to include: questions, suggestions, discussions
Example: To make suggestions, ask questions of discuss anything relating to Communities of Practices (COP's)