A good frame and fork, however if you get a more basic fork you can usually upgrade it depending on your preferences and budget.
A good set of tires, because you will notice a difference in corners, climbing and rolling resistance (Kenda is my choice brand)
A good set of tools, preferably ones you can take riding. Having cheap tools is nice until you're stranded and a tool breaks or strips itself. Good tools make working on your bike easy and fun. Besides, if you end up on a group ride or tagging along with others you never know what they may or may not have (I've saved too many riding buddies to leave my tools at home).