Not much. The game is won or lost on creatures. The problem is, those creature dice usually only have a 50%-67 chance of coming up as a creature. It is quite possible to have the better dice bag, and just lose because of poor dice rolls.
With only one buy, the best purchase is usually obvious. Since most spells don't gain you glory and creatures that score also allow you to cull your weak starting dice, you are usually best off buying the most costly creature you can get. The most costly creature usually scores the most Glory, and usually has higher defense and attack power. Many times, whomever gets an early 8 Quiddy starting roll and can buy the Dragon or other 3-4 Glory creature first will usually win.
The game is usually pretty short. This is especially true of the 4 player game, where the game ends on 12 glory. This doesn't give the dice bags much time to develop a strategy.
The Advanced and Expert rules on page 10 of the newest rule book do add some important strategic changes. With the Advanced rule, you can buy two dice per turn, that makes you have to decide on multiple smaller quicker scores (less risk of not rolling a creature), or bigger creatures that are more likely to survive. The Expert rules require you to cull the creature that scores. This rule makes the game take longer, and gives dice bag strategies more time to develop.