One important aspect is that two peers without port forwarding will never be able to connect to each other. This is important for torrents with low amount of peers. Unless a connectable peer comes in to essentially relay data by grabbing the content from the other person and then seeding to you, you won't be able to download. For popular torrents with plenty of connectable peers to go around this is less of an problem, it will only cause some performance issues.
One important aspect is that two peers without port forwarding will never be able to connect to each other. This is important for torrents with low amount of peers. Unless a connectable peer comes in to essentially relay data by grabbing the content from the other person and then seeding to you, you won't be able to download. For popular torrents with plenty of connectable peers to go around this is less of an problem, it will only cause some performance issues.