Yeah, balance tends to be a major issue in a lot of fighting games. An example is when the fastest character in the game isn't the weakest (see: Captain Falcon). And then the slower, stronger version of him (Ganondorf) has the speed of Mario with the power of Bowser.
Then again, Smash Bros isn't exactly known for being entirely a serious, competitive fighter with a complete focus on skill. Sure, there are 1v1 tournaments with top tier characters fighting with no items on Final Destination, but there's the random chance, chaotic, flashy "Holy shit what the hell just happened" element to it. That manifests itself not only in the items and other things that can be turned off, but certain characters' attacks (Peach's Forward Smash, Mr G&W's Judgment, Dedede's Waddle Dee Toss, etc.)
Another problem with balance in sequels is that, rather than upgrading the lower tier characters to match the higher tiers, they nerf the upper tier characters, with the result being that they're not as fun to play.
What I'm looking forward to the most about the game is the Subspace Emissary. I've always wanted a platforming game where fighting was a huge focus. What makes it even sweeter is the co-op modes they're throwing in. That will add huge variety to the game, and make it more fun even when there's a huge difference in skill between me and one of my friends. Normally, when I have a friend over and play Smash Bros, we play a good 10 or so 1 on 1 rounds, and then we want to play something else.
The one biggest flaw about Smash Bros (other than random chance things that you can turn off) is that at low percentages, sometimes your opponent can recover from getting hit by your attack faster than you can recover from doing it. Take Mario's Super Jump Punch for example. At low percentages, it hardly knocks enemies away, and Mario's unable to do anything else until he lands, leaving him open to attack from the same enemy he just hit.
One time, I hit a CPU enemy against the wall in Peach's Castle. They ended up jumping off the wall almost instantly after and jump kicking me almost instantly after.