In much the same way we can star questions that we want to keep track of is there a way (or can one be implemented) that allows us to star other users?
I imagine it being used in cases where one person really goes above and beyond in answering a question and the asker wants to mark the answerer (real word?) for easy recognition later.
I think the idea of StackExchange sites is that you should be voting on the answer of its own merits, not on the merits of who posted it.
Easily following a user would more open up the system for "gaming" where people follow their friend|enemies and upvote|downvote.
This is possible by going into a user's profile and looking at their questions and answers, but it isn't quite as exposed.
But people do come with baggage -- we are more inclined to view the answer from a person with 12.6K rep as more authoritative than from somebody with 126 rep. Even though the latter might have given a better answer. But the goal is to vote answers for their own merits.
Here's something I've seen more times than I care to think about: a user with a history of posting well-researched, well-written answers drops into a question and posts something lousy, inaccurate, hastily-written, unhelpful or just plain wrong.
Everyone has their off-days and blind spots, and past performance is no guarantee of future results. Apart from the rare instances where someone answers a question on a subject where they are the last word (say Steve Kondik shows up to answer a Cyanogen question or something), it's usually a bad idea to blindly trust someone's answer simply because they've given you good answers in the past.
So this is not likely to ever be implemented on any SE site.
To add to what Michael P. said: if you're really interested in a particular user's activity you can always subscribe to his/her user rss feed. The link is found on the user profile page, in the lower right corner.
