Annoying in a light hearted way (i laugh to myself when i see it)...cyclists with nearly flat tyres...i always say at work (to anyone that'll listen) when i see a half flat tyre..'get some air in them tyres'
Same with dry chains...'get some oil on that chain'
There's a guy on my commute, see him regularly but never spoke...his helmet is always tilted up off his forhead. Looks ridiculous.
The only time i ever thought 'you freekin idiot' was a few years ago. I rode up a hill at fair pace, overtaking a couple on reasonable quality, maybe 28mm tyred hybrids. I rode on, maintaining fair pace..and about 4 miles later i became aware someones on my wheel. Quick glance, its him. He's abandoned his partner to give chase, took 4 miles to do it, overtook me...then instantly turned round to go back and find his wife/partner.
All that so he could 'put one over me'. If i'd known, i'd have made it difficult/impossible for him. I remember thinking..'what the hell must your wife think as you dissapear off into the distance and abandon her'