+++ Zeromus is the hardest last boss in the entire series
++ First notable RPG with an actual storyline
++ Moderately Difficult
++ Still fun to replay after several times
++ Pretty sweet soundtrack
+ First game to offer a "Battle Speed" option which is essentially a difficulty adjustment meter
+ Kain is a supreme badass
+ Varied environments
- Most characters have fixed levels when they join or rejoin the party, sometimes forcing you to re-level them
- Pioneered the ever so useless last minute villain switch
- Takes forever to get your final party
- Edward sucks
Final Fantasy IV is the first great JRPG ever made, with very few flaws to speak of. In being one of the earliest RPGs with a storyline (distinguishable protagonist/antagonist, more than a few characters) it is the proverbial dartboard which was randomly thrown at and a lot of the ideas really work quite well. There's Ninjas, Karate dudes, Dwarves, the underground, a tower that reaches into the heavens, the last part of the game takes place on the moon, the summoning realm, a village of mystics, and everything else you can imagine.
The vast majority of these now somewhat commonplace ideas are quite functional in the scope of the game and pretty good for JRPGs in general, though the one major complaint I have is that the awesome Golbez is randomly discarded as the primary villain in the last few hours for no apparent reason. Instead the face of all evil Zeromus appears and while he is a great final boss the cliche that spawned has destroyed the plot of many a game. Consistency in villains is quite important to create a sense of dread as well as a sense that you're working toward some final goal at the end of the game.
This game is also one of the most difficult Final Fantasy games around (provided you play it at the fastest battle speed) and forces you to make quick decisions to kill particular bosses and occasionally experiment with elemental weaknesses. By the end the best strategy is pretty standard, load up on your strongest attacks and precast powerful heals to defeat most bosses, but it is fairly difficult to actually execute all of those attacks prior to dying in some cases. The Plague, a boss in the final area of the game, casts "Doom" (death after 10 seconds) on your whole party and that time will tick down extremely rapidly on the fastest speed, so you unload the requisite 20,000 damage in time to kill him. I succeeded at this the first time but I found the result to be pretty awesome:
By the time I got to Zeromus I was about level 58 (no grinding for Adamant armors or any such silliness), which is right about where you're supposed to be. Big Z's main attack Big Bang does 1800-2200 damage, sometimes enough to 1 shot Rydia and always enough to kill my team in 2 hits, and this fight is extraordinarily difficult without one of the optional summons (though apart from the Sylph cave there's no reason not to do the optional areas in the game and they don't leave you too overpowered by the end). Rosa's Cure 4 does around 1500 healing to all and Zeromus has about 4 times as much HP as any other enemy in the game so he will get off ~15 Big Bangs before he croaks unless you're ludicrously overleveled, so you have to use Rydia's Asura summon and hope that it keeps casting healing spells (random choice between Cure3/4 and Life), naturally the first few attempts went something like this:
There's not a whole lot else to say about this game, it's quite straightforward with a minimal (but still quite fun) amount of sidequests and it doesn't have some legion of followers talking about how amazing it is that a clown blew up the world because he was fucking crazy and how awesome that is. The plot isn't great but it's not terrible either and the gameplay remains entertaining to this day, albeit the first 20 hours prior to getting your main party can get to be tiresome without the magic of fast forward.
Final Score: 9/10