Okay, Samus is MUCH better than I give her credit for.
Her biggest saving graces, in my opinion, are....
1. Grab release opportunities
Samus has a very fast grab attack. Each hit recharges your moves, and this means all of Samus' moves are going to be fresh 99% of the time. That is HUGE.
Just as important is what you can do AFTER. A common kill combo I have is to simply grab someone, hit a repeatedly, then d-tilt them when they get out. You don't have time to grab them again due to Samus' start up lag for her grab, and often a character can shield the slower attacks such as d-tilt (sadly).
However, this gives Samus a very effective mix-up game. She can grab release to single jab to grab to grab release to charge shot to grab to d-tilt, etc., etc., etc. I wish I recorded all my games with Samus, but I only recorded a few against my little brother; I'll be putting them up shortly. Needless to say, she's got some nifty opportunities here.
The moment your opponent gets in the habit of shielding, you can grab, attack, grab release and grab again. When he isn't shielding and is near 100%-120%, you can d-tilt for the kill. This is extremely important to Samus, as it gives her a mix-up game that can't actually be "countered". Samus has an option for whatever you do, and it turns into rock paper scissors. That makes Samus deadly.
My favorite personally is the grab release to full charge shot ^_^
2. Her dair
Her dair has crazy priority. I down-aired Squirtle and Charizard both out of their up+b with ease. That is crazy. It's incredibly easy to use and Samus can ALWAYS get back.
A lot of characters can't really edgeguard so much as hold the edge then hit their opponent after they use their up+b; it's very hard to edgeguard effectively in Brawl. Samus doesn't have it much easier, but with her down-air, it gives you a low-risk / high-reward scenario every time your opponent is off the edge. If you are able to read your opponent, it costs them a stock. This ties in well with her next saving grace...
3. Her survivability
Samus lives a long time and can always get back unless you do something silly. What's crazier is you can DI straight down and hit Z when you are at ridiculously high % and you'll grab the edge with your tether, stopping your momentum. You can survive for a long time in this manner.
4. Her automatic combos
Samus can, with her u-air, fair, uncharged charge shot, bomb, and grab release set up automatic combos. This means, EVERY time you do X, you can do Y. They're not easy to pull off (if only her fair had a lower hitbox...), but if you do pull it off, it's a free f-smash. The ability to land a decent smash attack whenever you want is a boon not many characters have.
Since Samus' big problem is getting KOs, I imagine that if someone was willing to practice with this character enough and create/find opportunities to use those automatic combos.... she wouldn't have nearly as much trouble. Samus isn't crazy good, but she's definitely not a horrible character. I imagine she will be exactly as she was in Melee; a few players wlil be very talented with her and just completely destroy certain matchups and be a unique and difficult opponent.
Also, I hate to say this, but.... she might be a heavy ROB counter. I originally laughed at this, but Samus' dair and fully charged charge shot are ROB's worst nightmare. Since ROB can't air-dodge out of his up+b, he is completely vulnerable from above by Samus' dair. For the same reason, Samus can jump down and fully charge shot him for a KO. This is all theory, but I'm generally right about things with ROB....
ROB still has the advantage on the stage, but iwth as long as Samus lives and her ability to force anyone to approach by utilizing her zair.... that could be a tough nut to crack.