Personal Relationships – How many of the people in your mass unfollowing were individuals you met in person and exchanged Twitter handles with as a means of staying in touch? This doesn’t fit the simplicity of the blog analogy. In this scenario you followed up a real world handshake with a digital one. To wantonly discard all of these connections seems to be a social faux pas at best.
Laziness – The lack of nuance conveys laziness. Getting to the point where you’ve created too big of a mess to handle is one thing, but dealing with it by dumping everything is simply lazy. Would you do the same if your desk were overrun with paper? Might a smarter and more prudent approach have been to protect your updates to prevent or slow new following relationships from being created and make a commitment to invest an afternoon in cleaning things up?
Your Original Intent – Twitter is still new and evolving but I think it is safe to say that your (and other mass unfollowers’) original intent was not to create a mass mailing list. But, here you are with a big list of “followers” that does not accurately reflect reality. Unlike Loic (founder of Seesmic and Le Web), who most likely followed back the majority of his large number of followers in an effort to be polite, I suspect that you and Joel (and I as well) actively followed people with the hope of reciprocal follow backs. What I mean is, the huge number of followers you have compared to the small number you now follow is not an accurate representation of your influence nor the contribution you have made. I know it wasn’t your intent to misrepresent yourself but the numbers are now fiction. They make it look as if you must be someone who is so important and creating such amazing content that everyone follows you but you only follow back a select few.
Value Exchange Fraud – Unfollowing people is fine, we all do it for different reasons. But dismissing everyone could be seen as a form of fraud. You still have the value of that person’s follow with no exchange of value in return. Now, if you are producing tremendous content, are a person of some fame, or a business leader as mentioned above the value exchange may still be there even without the follow. But, I suspect that if you sent a DM or @ reply to each person who is following you and let him or her know that you unfollowed but request they continue to follow if they feel that they are receiving value in return you may just be disappointed in the result.
People are not RSS Readers – When I add a blog to my RSS reader it is completely anonymous, you have no idea if I read your blog or not in most cases. But, with Twitter the connection is overt and public. You see me and I see you. It’s a relationship.
Disregard for the Impact on Others – How many times have you looked at who follows a person when considering whether or not you should follow them? Did you dump a newbie or two who were excited about this new medium.

I have no love for the follower construct. I actually see it as unnecessary but the unfollow everyone approach just seems extreme and lacking in authenticity.

Gerald Weber of Search Engine Journal clued me into Joel and why he think it’s a terrible idea: http://www.searchenginejournal.com/unfollow-friends-twitter/

On Joel’s blog, I’d like to highlight a particular comment he wrote in response to Gerald:

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I’m sure you subscribe to multiple blog feeds, and I’m sure you have many people that subscribed to yours (maybe because you subscribed to theirs and left a comment at one point) but that doesn’t mean that it would be unfair to go through your feed reader, and unsubscribe from some of the blogs that are not that relevant and take too much time to keep up with.

You would still be sending your blog posts out to all of your subscribers, and I doubt you would write a post about why you unsubscribed from a load of blogs. It doesn’t mean you should quit blogging for that blog, and start another one.

It’s not at all a one way communication when I tweet. If they follow me, and I tweet a message, they can reply, and I’ll reply to them.

Same with blogs. If you have thousands of subscribers, you can’t be expected to subscribe to thousands of other blogs. It just doesn’t work like that. It’s a two way communication because people can leave comments.
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– http://joeldrapper.com/unfollow-everyone/comment-page-1/#comment-2378

I couldn’t say it better myself.