Maybe I’ve been on Twitter to long and lost my patience a little, but over time I’ve developed a rule set that, when violated, contributes to my decision to unfollow someone.
Some of these offenses need only happen once, whereas others take a few occurrences (with no valuable content in between) before I unfollow.
1) You Publish a Paper.li Daily
These have become so popular that I finally had to check it out for myself. I “published” one of these “dailies” expecting a surge of readers to justify why so many normally sane people have jumped on this bandwagon.
I can confidently say that no one reads these things. They are the vanity press of twitter, but more annoying because you’re bugging people to read it.

2) You are obsessed with startup/entrepreneur porn
Eric Ries, lean startup, minimum viable product, VC, pivots, widening the funnel - we get it! You’re an entrepreneur!
Perhaps it’s just the set of people I follow, but the twitter echo chamber is never more acutely felt than when a dozen different people tweet “Top 10 Tips For Raising Your Series A” over a two day period.
Is everyone out there chasing a VC?

3) You tweet your Foursquare checkin at Starbucks
Remember when Foursquare first launched and everyone spammed their timeline with all their checkins? That was decades ago in internet time. Now we only share a checkin on twitter if we’re backstage with Alice Cooper or summiting K2.

4) You live-tweet conference sessions, or worse, the WWDC Keynote
You’re not a journalist. We have people who cover these events for a living - and do a far better job of it. A single 140 character quote will not capture the lightbulb that went off in your head during that legendary mobile games monetization panel. And it’s very unlikely that you’ll break any news by live-tweeting the video stream of Steve Jobs’ “one more thing…”

5) You tweet hyperlocal news
I have four kids of my own, so believe me - I love kids (most of the time). But tweeting an Amber Alert (missing kid) is about as effective as faxing your missing cat poster around the world.
Even worse are traffic jam notifications. The internet’s a big place - it’s not very likely that timely detours to avoid Chicago traffic matters to me in Vancouver.

6) You are too far down the web’s linguistic rabbit hole
Thankfully, terms like FTW or Fail have been mostly retired and are only used by those truly “LTTP.” But too often we’re still reducing ourselves to toddler-level language with expressions like “something + something = awesomesauce” or “just sayin”.
Also, if you think bacon, unicorns or rainbows are the height of humor, I have nothing in common with you.

7) You’re a one-dimensional Twitter personality
I’m sure we’re all a bit guilty of this - this time of year I tweet a lot about my local hockey team (Go Canucks Go!) - but if you repeatedly bang the same drum over and over again, you’ll lose me.
Vegans are especially single-minded. But worse are those who devote nearly every tweet to promoting their project, company or themselves, going so far as to RT any tweet they’re mentioned in.
I believe the rule is 1:10 - i.e. for every promo tweet, throw in 10 no-agenda tweets.

8) You make a mockery of hashtags
Were hashtags ever useful? These days they’re used almost exclusively as the punchline of a joke tweet. Click on these haha-hashtags and you won’t find any other tweets using them, of course.

9) You don’t converse
If I’ve @ mentioned you several times and you’ve never replied, you’re not using twitter properly. If I look through your timeline and it’s mostly broadcast tweets with very few @ messages, then shame on me for even following you in the first place.
Yes there are stats that show Twitter is essentially a broadcast medium - that most people don’t contribute and instead watch those that do - but there are people who do engage and when they do, reciprocate.
10) You craft your own philosophical or business quotes
The world doesn’t need any more mediocre quotes, especially in the realms of philosophy or business. So don’t waste your time trying to author that perfect bon mot, then going so far as to attribute it to yourself.

So… what makes you unfollow a tweep?