Fuel economy, comfort, safety, less mental stress, less wear on the vehicle, vehicle capability, unfamiliar with the road, looking for a turning etc. The limit is the maximum not a requirement so I don't understand the hostility towards drivers who aren't always doing the maximum. I can see the problem when someone is significantly slower e.g. half the limit in good conditions, but most here seem to be complaining about drivers doing 40ish in a 60. Just because you and your vehicle are willing to travel faster, doesn't mean everyone is.
The worst offenders for driving slow that i've noticed recently, people with uneconomical cars. Why buy a car with a big engine then drive everywhere slow, if your that bothered about saving fuel get a diesel...
Your statement is as daft as me saying that if you like driving fast so much why don't you drive a Koenigsegg everywhere? Maybe the "offenders" need the practicality of the bigger car. Diesels tend to cost more initially which low mileage drivers won't recoup. On a tight budget it is quite often cheaper upfront to buy a big old exec barge rather than an overpriced poverty spec small eco family car. Why shouldn't one get the best mpg out whatever engine they have?
The comments berating the new driver about pulling out into traffic seem a bit one sided. If the oncoming traffic is significantly speeding and there aren't other opportunities then I don't have much sympathy for the speeders if slower traffic pulls out in front. Hint: maybe the lower limit is in place to allow traffic to pull out safely! By the same token presumably some posters think that pedestrians deserve to be run over if they don't allow enough space to cross a road in front of a driver doing 50 in a 30 (even though there would have been enough time at 30). Although in the particular case mentioned it does sound like the new driver was at fault
I drive according to the conditions and in some circumstances that does mean going slower than the limit. In good conditions I would prefer be at or (in theory m'lud) a little over the limit because the car and I are capable. With a big torquey diesel I enjoy plenty of great overtaking opportunities. However I just don't seem to get as bothered by slightly slower traffic, even if I can't overtake.
Now it's a different matter if we talk about people who tailgate, are too lazy to indicate (esp on roundabouts), don't show any etiquette at lane reductions, block box junctions, generally drive erratically...
Last time I drove to Gatwick, had some proper a-hole sitting on my bumper even flashing his lights and sounding his horn at me for about 20mins. Before someone says "don't be selfish pull over", realise that this was in heavy rain, heavy traffic in all lanes (nose to tail standing still at times) and most significantly I spent the entire time behind the same other vehicle anyway i.e. it's not like prevailing traffic was going faster or there was anywhere for him to go, without him harassing every vehicle on the road to get out of his way.