Damnable FB references and knee-jerk "RSS feed" ducking aside...
A 'follow user' feature would provide another data stream for all the reputation algorithm trickery that makes SO/SE sites so powerful.
Lifehacker added a system like this to provide smart filtering in their comments due to the general overload and lack of hierarchy. It provided a disincentive to post meaningless "first" type of comments since followed user comments would rise and it empowered individual users to filter comments to their liking (e.g. snarky and funny vs. information rich, etc). LH also used aggregated follows as a way to promote certain users generally.
Personally I would love this feature. When I stumble upon someone who makes clear, insightful replies replete with code examples I would like a way to keep track or somehow highlight said user. Looking at a bunch of RSS feeds is not helpful - I don't use RSS feeds to track SO activity and don't necessarily want to know everything a person does all the time. This is a cumbersome non-feature for this specific use-case. UX-wins of a 'follow' feature could include:
- When viewing individual questions the comments/answers by users you follow could be highlighted in some way (badge, literal highlight, etc) - esp. helpful if you are scanning for quick solutions, adds "trust" filter
- In search results, questions where followed users are involved could be highlighted in some manner
- http://stackoverflow.com/usershttps://stackoverflow.com/users could offer a 'following' sort (tab).
- Like the user tab 'favorites' for referring back to questions you bookmarked, a 'following' tab could list questions involving those you follow, which could then be refined by 'questions' 'answers' 'accepted answers' etc.
- To avoid the "club mentality" some seem to fear and to avoid social-network-ishness...simply to not display 'following' lists publicly or reward following type behavior like showing a number of followers a person has, etc.