There's something of a tradition on SO of using CW for polls: questions with a finite number of answers where users are expected to vote up the existing answers rather than posting duplicates, edit in additional information where appropriate, etc.
Examples: Hidden Features of C#?, C++ Urban Myths
Another sort of question that's frequently seen on SO (and also often marked Community Wiki) is the "What do you think?" question. These aren't so much polls as they are discussion threads: the phrasing of the question invites each reader to share his own personal experiences and opinions, and discussion frequently thrives in the comments as well. While you will find many of these on SO (indeed, this category includes most of the most popular questions on the site), they tend to be more readily closed, as they have little to offer casual readers beyond opinionated arguments.
Examples: What is the worst programming language you ever worked with?, What are the worst working conditions you have written code in?
As should be clear from the examples, the deciding factor is public opinion1, not any set of concrete rules: if enough people like your question, it'll either stay open or get re-opened; otherwise, it'll be closed and eventually deleted. No one person2 can say for sure which outcome will befall an individual question...
1Ok, so if a single moderator likes it then they can keep it open, and if a single moderator hates it, then they can delete it outright... but those tend to be edge cases.
2apart from moderators1