Crafting in the lower worlds is rather worthless, at least as far as gear is concerned. However, crafting gives some amazing gear options starting at level 55, up through 95. Crafted gear at those levels is second only to Waterworks or House of Scales gear, both coming from dungeon drops. You should do your crafting before you reach level 55.
Be sure you also get a card crafting table (useful for transmuting one type of reagent into another).
For doing quests, you need to decide if you want to do all side quests, or only the important ones. If doing all, start at the lowest world and complete them one by one 'til you're done. If skipping the optional ones, then always look out for the rewards. If any quest shows a spell, training point or pet as its reward, DO NOT skip those.
Some quests you must do. Any quest from Zeke or your professor are useful for additional spells. The main quest line in Grizzleheim will lead you to a spell at level 35. Continuing in Grizzleheim and completing Wintertusk up to the start of Nordrilund, will result in several spells, a minion, and all your school pets past level 48.
Any quest that has stars bordering it is a "main" quest for that world. You want to finish all of those (exception is Wysteria, which is truly optional).