Torrenting requires at least one side (either seed or leech) to have an open port. If you don't have an open port, you'll be able to torrent but will only be able to leech from seeds that have their ports open. So it will potentially limit your torrenting speed and/or completion. For instance, what if there are 10 seeds, but none have open ports? You'll sit there being able to see 10 seeds, but your client won't ever actually connect to them. So you'll be stuck with a torrent that looks like it could finish but never will because you can't connect to the available seeds.
Of course, if you're only torrenting popular content with thousands of seeds, you'll probably be fine. The chances of some of them having an open port are pretty good. But for smaller torrents with only a few seeds, you may find yourself stalled even when seeds are available.