Apart from the continuity issue (see below) the other thing that bugs me a bit about fantasy fiction is the passage of time. Vast amounts of time.
Take David Eddings' Belgariad/Mallorean saga for instance - we have a central character over 7,000 years old and others who have been around almost as long. In Terry Brook's Shanarra series we have over 500 years between events with, again, thousands of years of history going on in the background. And yet everyone is still living in "medieaval" times. This goes for most of the epic sagas out there on the shelves.
Does no-one in fantasy land ever watch their kettle boiling and have a James Watt moment (yes I know that's probably apocryphal, but you know what I mean)? Do the people who build the sophisticated locks for city gates, construct the siege engines, forge the weapons and armour never tinker with clockwork or wonder how the transport system could be improved?
When you look at how life on Earth has changed over just the last 1000 years, it seems odd to me that people we are supposed to believe to be as intelligent as ourselves would not have come up with a few of the innovations we've seen over that period. Sometimes you get pseudo-explanations such as magical accidents making events repeat themselves rather than progress, or evil wizards stamping out change, but still. It doesn't stop me devouring trilogies by the truckload but at the back of my mind I'm always wondering how life would be better if the hero had a jeep or even a bike at his disposal for those long quests in search of enlightenment or a magical wotsit.