4. Avoid major graphical glitches.
Errors such as garbled sprites/FGs/BGs, message box text screwing up layer 3 items, floating/stacked/cutoff tiles, and general graphical ugliness are frowned upon. Furthermore, don't forget about glitches which involve the sprite memory - the graphics of the sprite will 'disappear', but the sprite will still be there, and can unfairly hurt Mario.
7. The player should not be able to enter Level 0 or 100 from the Overworld.
Level 0 and 100 are the Bonus Game levels, which are handled in a special manner that makes them 'endless' if you enter one from the overworld. This causes the player to be trapped forever. To prevent this, check and test your overworld events and levels to make sure that they lead where you want them to.
9. Keep the hack at a fair difficulty.
Avoid issues such as death traps after the goal, blind jumps, forced damage, excessive enemies, places where you can get permanently stuck, excessive 3-UP moons, projectile sprites (e.g Bullet Bill) placed directly into a level instead of using the correct shooter/generator sprite, etc.
11. Have fellow users beta test your hack before submitting it.
Your hack should be as good as you can make it before you submit it to be featured in the hack database. A good way to make sure you have a quality hack is have it beta tested. Have your friends play it and give you feedback, or recruit some beta testers.