What I learned from X-2...
People are willing to play Charlie's Angels meets Barbie Dress-Up, long as you call it Final Fantasy.
If the prequel to your game began with the complete destruction of one civilization, the aproporiate way to begin the sequel is a pop concert.
Changing your clothing takes 10 seconds.
Dress-spheres only change your clothing if you're one of Brother's Angels. Otherwise they change your entire body completely. (The game's intro proves this quite well)
What I've learned from FF7
Cloud's pretty useless, and a stupid whiner to boot. If you replace him with someone halfway competent, Aeris would be alive and Sephiroth would never get his paws on the black Materia.
The most dangerous villains are always the ones with an Oedipus Complex the size of the goddamned moon.
Following the above point, Sephiroth is a long-haired Norman Bates, a generic momma's boy, and one of the worst villains ever.
Further following the above point, fangirls don't give a crap about any of it because he shows his chest off all the time.
Stunningly, if the cast of characters is a bunch of annoying people, stereotypes and moody emos, the two characters with the strongest personality can be an abusive potty-mouth and a talking tiger.
What I've learned from FF8
A unique system that increases the personality of summons and makes spells NOT be completely freaking useless halfway through disc 1 will only get hated by everyone.
A hero that starts out silent and moody, but grows out of it as he learns to protect others, will be hated by everyone.
You can make a sniper that cracks under the pressure of his job, something that should be dramatic, and STILL make him annoying.
The mysterious magic girl with the important-to-the-story pendant is always annoying and useless. This lesson was taught by 7 and 9 as well. In 10, there's no important-to-the-story pendant, and what do you know? Yuna's annoying, but also the game's strongest character by far!
Monsters levelling with you CAN be done right. Take THAT, Oblivion!