One of the game's biggest thrills comes from the possibility of one of your characters dying and the chance that it might be possible to get through that battle without that death, if you started over just one more time. It is important that the NPCs have more to talk about than just their jobs or your mission. It is better to have too much dialogue than just the bare minimum, as talking is essentially optional. It is in this aspect of the game where your skill as a storyteller again comes into play. The plot is another way to hook the player and it has to be at least fairly pedestrian, sturdy, and complete, before you start making it elaborate.
Innovation can crop up in any part or parts of a scenario, and it is difficult to define.
Most people can spot it when it happens. If a player only wants to play for a couple of hours, she doesn't look for a complete scenario which can be played in that time, she just plays for as long as she wants. Several towns, dungeons, and outdoor sections are minimum for a good scenario. Riddles and puzzles are only fun when the player is able to solve them and the intended path should never be impeded by them. Custom graphics can look great or they can look like crap.