The Weather- Sometimes it's hot and that engrosses you (not the correct usage) and sometimes it's cold and sleeting and you are soon covered in cold sleet and your toes are number than your fingers (though both ten hopefully?) and your nose is even number (2..4..6..) and you question why you're outside and/or if you will ever be able to experience sensations of warmth ever again. Other times it's nice. Then there's wind, which is a subset of weatheriness which theoretically could be at your back (note: never happens) or bluster you face-wise until you're traveling negative 5 miles per hour. Basically, if even half the days fall within your weather "don't mind biking in this zone," half will fall out of it. But that's ok! Biking in conditions that aren't your ideal ones is still better than climately controlling yourself in a gilded, gas guzzling birdcage. Probably.
Smells- Sometimes you get to smell good stuff, like freshly cut grass or baked bread. [I will simply ignore the vast majority of other urban smells that you are more likely to encounter. Also, I've had occasion to bike by an honest-to-goodness fish market on a summer afternoon and you shouldn't do that sort of thing]
You can hear other people and they can hear you- Aren't people the worst? Don't they talk about the most banal and stupid stuff? Aren't they all like "something something Senator Ted Cruz something something" all of the time and isn't that just the worst of all sorts of ear pollution? Aside from actual pollution which might infiltrate one's ears. However, the rolling eavesdrop is easily one of the best parts of any bike ride. Extrapolate whole relationship histories from snippets! Reverse engineer the setups of bad jokes from the punchlines of those very same bad jokes! Marvel at what strangers have to say about other strangers in front of other strangers. Oh, and be careful if you have the tendency to talk aloud to yourself while riding. More than once I've said "well, that was some terrible driving' within open-window shot of the offending driver.
Endlessly covering the same terrain- This isn't really unique to bike commuting. It's the same for drivers and bus riders and metroists repeatedly revisit the same subterrain. So, it can get boring. But I find that commuting my bike really attunes me to when things change- stores and restaurants open and close, potholes appear or are filled, road stripes are restriped- you know, exciting stuff- and it's nice to have one slice of the city that you know really, really well and can feel live and breathe under you and around you as you pedal through it.
Jerks- Ok, not really. I haven't come to love jerks. I'm pretty meh on jerks still.
Hills- Riding uphill is great. Riding uphill lifts your spirit. Riding uphill makes you a better person. Riding uphill removes difficult stains from your clothes. Riding uphill can julienne vegetables. Riding uphill provides better 4G and LTE service than any other carrier. Riding uphill can beat a shark in some kind of shark fight. Riding uphill is good for the soul. Riding uphill gives you Stockholm Syndrome.
Red lights- The knock on bike types for some reason is that we are sufficiently in awe of the amazing traffic control powers of traffic regulatory devices. This makes us just like every other kind of traveler. Drivers tremble not at warnings that they oughtn't, no matter how much they might want to, turn right on red. Some don't, some do. I've yet to see groups of pedestrians attempt to blockade or claw back an intrepid Icarus from risking it all against the Stern Red Hand. But anyway. Red lights seem like they should be annoying because they slow down the march of progress, but they really aren't that bad. They give you time to sip coffee or water or Sport Drink or to catch your breath or to [single sappy people only] lock eyes with a stranger and fall deeply in love or to look at someone's sneakers and think "do I want a pair of sneakers that look like that?" Other things you could do at red lights include checking the time or trying to remember some trivia factoid. Red lights are the punctuation marks in the sentence that is your bike commute. May it be a life sentence. Wait, what?