#1) Slow drivers.
~I mean seriously... lead, follow or get the hell outta' my way!
#2) People who come to damn near (or completely to) a complete stop when making a turn into a driveway or wherever. ~I mean come on! One only needs to slow down to about 5 mile an hour, no less than 3 and make the turn while continuing to slow down while your in the turn.
#3) People who swing out wide into the opposite side of the road to make a simple turn. Unless you're driving an 18 wheeler, it's not necessary.
This is why I'd rather (yes even at age 30, almost 31) ride my 2008 GT SCOPEDOG BMX bike everywhere if I could, than to have to drive... could just jump a curb, zigzag in between vehicles and take a side road, go down an alley, ride down an embankment leading to a dirt road, etc. But sadly that just isn't feasible most of the time.