Most of that was in my general knowledge, however, writing it up in an article sure helps, especially for newbs. Let me add one all too common mistake.
9) Balance items, shops, spells with the level of the scenario.
If you make a scenario for level’s 10-15 don't make steel blacksmiths available at every corner. In the VotdT most blacksmiths sold leather, bronze and a few iron armour. A good normal sized scenario should have no more than four very powerful items and a handful of other great items in hidden locations, side quests or tough encounters. Balance the spells with the requirements of your level and the scenario. Don't make a wizard teach Arcane Summon (3) if your mage is at level 7. etc. Also, keep track of costs. At times, shops can be rendered useless and mere decorations for your scenario if prices are too high. The ludicrous prices are getting too common and in the end most stuff is never bought because better items are quickly available on the wandering monster outside.
