Twitter is subtly trying to encourage users to add topics (in the form of hashtags) to their tweets. It has advantages at both ends - from a user perspective :

Twitter is subtly trying to encourage users to add topics (in the form of hashtags) to their tweets. It has advantages at both ends - from a user perspective : you can pick and choose topics to follow and from an advertiser's perspective : you now have more contextual information about the tweet. It might help you serve better ads and recommendations. However, if this encourages such a behavior there is a very good chance of lists becoming obsolete. Rather than lists one would simply create a timeline of users (with a given hashtag) and share it to the world.