iPhone, Android, and Mac clients. Maintaining as well as improving the web interface and the API. Lots more scaling - people check Twitter way more than they do Wordpress-hosted blogs.
I'm sure there's more, this is just what came off the top of my head.
That client would then touch a lot of other people though, even if it was primarily the one guy. Changes that are rolled out on the platform would need to be considered in terms of this app along with the all other platform apps.
Sure when independent a single guy can sit there and create something like that solo, inside Twitter though there would be a lot more process and interaction to achieve the goals.
I'm sure there's more, this is just what came off the top of my head.