It also enforces actual use in the wild of tags, since no evidence of a tag will exist without it first being used in conversation. This means that representing channels in tagclouds across the site that grow and fade over time, and are contextual to all of Twitter or to a single user, is the ideal interface for displaying this information.
Hashtags have the added benefit that they won't show up for others if they're not used.
If you look at which hashtags are being used (trending), you get a taxonomy of micro-contexts, ranked by popularity, with which you can navigate Twitter. All from the bottom up.