From a pedestrian's point of view, they're too quiet and too fast.

From a motorist's point of view, they're too slow. Oh, and motorists tend to view the few cyclists misbehaving on the roads (e.g. cycling onto footpaths to avoid red lights, riding two abreast) with significantly more vitriol than other motorists misbehaving on the roads. In law, cyclists are expected to give pedestrians priority, and in reality they'll give motor vehicles priority as well, so they're at the bottom of the hierarchy. They're not allowed on footways (courtesy of a law dating back to the 1870s, when motorised traffic was non-existent). As another silly example, mobility scooters come in two classes: one kind speed limited to 4mph, which is allowed on pavements, and one speed limited to 8mph, which has to use the roads.

Heck, even traffic engineers seem to have something against cyclists... how else would you explain these examples? Then of course there's the variety in official signage. On-road cycle routes may be painted with solid lines (motor vehicles are supposed to be prohibited from entering these at all times, although it's not too hard to find motorists mistaking these for narrow parking bays...), dashed lines (motorists are only supposed to enter if unavoidable) or even no lines (just cycle symbols painted on the road at intervals).