One of the biggest additions in Super Smash Bros. Brawl is easily the Smash Ball and the character-specific Final Smash attacks that the glowing orb unlocks. Because of the incredible power of Final Smashes, the Smash Ball radically changes the flow of a match each time it appears. The item's movement is quick and erratic, forcing anyone who wants to use a Final Smash to drop what they were doing and chase after it, knocking away rivals while trying to grab Smash Ball themselves. The winner of this miniature competition gets access to a unique move that, if used properly, can radically change the course of a battle. I think the whole thing is a lot of fun.
Here are some things I like about the Final Smash system:
1) The Smash Ball's erratic movements, and the fact that it must be hit several times for someone to acquire a Final Smash, are both really good ideas. The chase these elements create makes it feel like whoever acquires a Smash Ball has earned it, and it makes the whole system is fair to every player. If a Smash Ball was just an item laying on the ground that could easily be picked up, it would not be as exciting, and it might lead to people feeling that luck was more of a factor than skill in the game,
2) I really like that Final Smashes are character specific. In previous iterations of the series, all of the most powerful attacks were items, mostly rare Pokemon, and were not associated with characters. Final Smashes put the emphasis back on the characters, which I think is a good thing.
3) A person can be awarded a Mercy Final Smash if they are badly losing a match. When I first heard about this I didn't like the idea, but after seeing it in play I changed my mind. This only occurs if a player is really far behind in score, usually so far that a single Final Smash will not be enough to turn their luck around. Even so, it gives that player a small hope, which can lead to dramatic comebacks (or even just a good attempt at one) and more interesting matches. Of course, realizing you were just given a Mercy Final smash can be disheartening as well...
4) I think that it is a good thing that you can knock a Smash Ball out of a character and prevent a Final Smash. It means that even the most powerful attacks can be countered, and rewards skill and aggression. However, this also leads to some imbalance, since there are many characters for whom this is a severe weakness, such as characters who need to carefully position before using a final smash like Ike or Meta-Knight, and others who don't need to fear it at all, such as transformers like Bowser or Sonic who can use a Final smash immediately.
Anyways, on to a few character-specific observations and comments.
I like the idea behind Samus's Zero Laser. The concept of a Final Smash that is somewhat more powerful than others (which I think the Zero Laser is), but can't be used often because of the character switch, is a solid one. As a person who heavily used Samus in Melee, I don't like Zero Suit Samus much, so I am not terribly happy with Samus being the one given such a Final Smash, but that is just a personal annoyance more than anything.
I think it was a serious mistake to allow any repetition in Final Smashes. Having three character all use the Landmaster Final Smash just emphasizes the fact that the Star Fox characters are too similar. Ness using PK Starstorm, a move he never learned in Earthbound, feels a bit forced, especially when Ness doesn't even use his own signature PSI ability (PK "Favorite Thing"). The fact that Toon Link uses the same Final Smash as Link feels like a wasted opportunity (maybe the very strong whirling slash from Minish Cap would have been better?).
I really like the cinematic Final Smash that Captain Falcon uses. I would have liked to see a few more that did something like that.
I think that either Marth or Ike should have had a Final Smash that emphasized that they were leaders of armies. King Dedede and Pit both summoned large armies of minions, so I see no reason that it wouldn't work for Marth to do so (I say Marth because his move is a lot less impressive than Ike's). A rain of arrows and spells followed by a Pegasus Knight charge would have been a lot of fun to watch.
I think it would have probably been more interesting if the Pokemon Trainer summoned a special Pokemon rather than use the Triple Finish Final Smash. It would have been a good way to showcase a powerful legendary Pokemon (like Mewtwo) at an impressive level of strength.
Other than that, I guess all I can say is that I generally like the various Final Smashes. They are a lot of fun.