As per the thread I ignorantly created here, this feature is pretty much a smarter version of something I was thinking about. Just voted it, hope someone can pick it up again!
I would just add one option to what has been described already (which I would recap as “filter items and only let new ones through”). That would be an option to “filter all results out on first run” so that only new items coming in after activation of the workflow would be processed. This way it would be easy to have workflows running on “incoming emails in Gmail”, for example (without having to resort to imap setup if you have set up Gmail with OAuth already). In other words, it would turn a cron node + polling node combo into a pseudo-trigger to start a workflow when real triggers don’t exist or aren’t possible.