Every final smash seems to go into a kind of category. This may prove unoriginality, or just go along with things like how Ike and Kirby share similar Up+B styles and Peach and Ike share rather similar counters. Note this is just with what info I know, and I don't want to come off as all knowing compared to the pros that may be here and have played it at demos.
Forward Blast: Mario, Samus, and Pokemon Trainer all use a general forward blast. Mario's covers a widening area, Samus apparently takes a lot of damage and forces a character swap, Pokemon Trainer uses three weaker attacks of differing range that, when close, will take a large amount of damage. Zelda's Light Arrow may count as a strong piercing very thin Forward Blast.
Flying Transformation: Sonic, Pikachu, Diddy Kong and Yoshi transform into immortal flying death machines. Sonic flies at hyper speeds and knocks around characters, but seems rather hard to aim with or stop. Pikachu seems a bit slower and perhaps stronger, seemingly not knocking around if you land on a character, but probably is not as good at spreading damage everywhere. He eventually slows to a stop, which, if it slows near a character, will seemingly doom the victim no matter what. Yoshi has free flight and immortality like the other two, but focuses more on a two move attack set of constant flames or strong dodgable fireballs. Diddy gets the ability to fly above the crowd and shoot highly explosive peanuts, but also risks giving the enemy a healing item should he miss and they notice. Snake may or may not count here, as he is unable to be damaged, is flying, and can pelt his foes to his choosing with possibility for error.
Super Transformation: Bowser, Wario and Fox temporarily transform into a super strong form that is still generally stuck by the laws of gravity. Bowser and possibly Wario both can take damage during these forms. Bowser becomes an unflinchable presumably unthrowable death machine, who's size leads to distance attacks still racking up damage as an easy target, but also means no character is really safe. Wario stays generally the same, with an enhanced moveset, with apparent special properties like hovering/flight and the like, and super strong Special moves. Fox gets into a deadly tank who's nearly/guranteed one hit KO is tough to hit, but is undamagable, and includes (infinite?) hovering and rolling capabilities.
Assisting Attack: Lucas and Pit summon a supply of deadly but dodgable projectiles, while still being able to attack, allowing things like grabbing your enemy to make an easy target. However they are also assumedly able to get KOed by those who manage to evade the enemies(and any general bad luck on the user)... Lucas summons a storm of meteors, which rises in count till probably being near undodgable. Pit summons a one at a time character aiming Centurion army that I believe is decently fast and dangerous, especially in a one on one one battle where you can grab a character just long enough to get hit. Dedede's Final Smash also is similar to these, though from what I can see, the speedy Waddle Dees sweep characters off more then they do lethal damage, the Gordos however, probably do do good damage.
Focused Strong Attack: Ike, Link and Meta Knight lock on to a character/s and deliver a fierce attack. Ike carries the hit opponent up, does a sweet combo, then slams them down with an explosion, also damaging others that may get in the way. Link focuses on one character and does massive damage, though I'm unsure if his final hit hurts any who may also walk in the way. Meta Knight does a single fierce strike on any characters hit by his somewhat short range cape swipe, but may also hit more then one.
All Encompasing Attack: Kirby, Olimar and Peach do an attack that affects everyone no matter where they are(in Peach's case, they have to be on the ground I believe). Kirby gathers up all the characters, does damage, then launches them off along with items that can be used by both him and any survivors. Olimar flies away as the remaining characters take damage, though I'm not sure how much, or if it changes depending on character count or if that simply meant "50 percent on one person vs 50 percent on three" or the like. He then has the chance to deliver a possibly finishing blow with his ship. Peach simply puts everyone on the ground to sleep from what I can tell, with a longer sleeping spell happening to those closer to her. She then gets the oppritunity to charge an attack, heal up, go for other items, or let the sleeping foes fall off a moving stage.
OTHER: Zero Suit Samus returns to her stronger but less agile Power Suit, apparently also damaging those close to her when it reforms. Does this do a lot of damage, or is the Zero Suit considered a punishment in a way, for those who don't know how to take advantage?
This is my analysis of the Final Smashes. Most Final Smashes are, in some form, possible to botch up and miss, even those who don't may leave the players not just alive, but able to take advantage, be it stealing items intended for the user, or using one enemy's Final Smash to take out someone else, claiming the KO for themselves. As someone posted above, there are also other nuances that can serve to spread similar ones apart for a more skilled player.
edit: ... I knew I forgot someone, I forgot the Ice Climbers! They are pretty weird, they seem to do damage somewhat, but I don't know if the Iceburg is constant damage or just occasional, they can freeze the enemy with it, and it causes the entire stage to become strange and slippery to everyone but them.
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