Though I should point out, 'less valuable' in terms of information. If you use Twitter as a gossipy chat service this isn't quite so much the problem. However, I prefer to use it for information purposes, and as a result I am fairly careful about who I follow. My figures, which are very modest I hasten to add, show that I am currently following 108 people while being followed by 232.
When I started using the service I trotted around to find people to follow, either via my email system (Twitter can check for you) or the names of key figures that I am aware of in the industry as well as particular information related friends and colleagues. So far so good, and that got me a fair number of people to follow. As you use the service people tend to find you - either by search, or by seeing recent Twitter posts linked to blogs and so on, or they'll see that you've blogged about their service and decide to follow you (which is how I get a lot of my own followers). It then becomes necessary to make a choice - follow or ignore them. You could choose to follow everyone of course, which is great for the macho 'look at my figures', but not so good for getting any useful content, unless you combine your use of the service with any of the myriad of add on services that allow you to create your own groups.
The method that I use is fairly simple -
a) Do I know this person myself? If so, it's almost a certainty that I'll add them.
b) Do I know of them? Again, if the answer is yes, they'll get added.
c) If I don't know them, what does their profile say? Everyone can add a small profile piece about themselves. If the profile is blank or not informative, they tend not to get added.
d) How many tweets on the first page of their profile are useful to me? If I start looking, reading and clicking almost instantly, they'll get added. If not, it's unlikely that I'll add them.
e) Are most of their tweets replies to other people? If so, and if I don't know the people they're replying to it's a waste of my time to read their stuff.
f) Are their tweets written in a language I can understand? That means, if it's not English, I don't follow them.
g) Are they using the service to blatently promote their services or blog? If there's a bunch of 'oh, please vote for me, or digg my post' it's an instant turn off.
h) Do they have a profile picture? This is a very minor point, but if there's no profile picture it's a slight nudge towards the no-follow group, but not crucial.
All of these criteria take about 20 seconds to work through, and since I'm not swamped with followers it's not a real issue. So, if you follow me, and I'm not following you, check out a-h and see which category you fall into! It's also important to remember that it's not personal, and shouldn't be seen as any sort of criticism.
As for what I write - well, that's probably a good topic for another post at another time!