Okay, I've actually played Heavy Brawl now. A few hours against the CPU, to be specific. Here are my observations.
1) The CPU has no frigging idea how to play Heavy Brawl. So keep that in mind as I give you my other impressions.
2) Link is unplayable. And I say this as a long-time Link main. I mean, wow.
3) Samus is pretty cool, though.
4) When you knock people upwards, you don't have to wait as long to be able to try to hit them again. Which is...convenient, I guess. But I fail to see how it helps combos. It's not like the hitstun is any less. The other player can still jump away or airdodge through you just as fast as before. There are a couple of new chainthrows and combos, but it's all very character-specific. A CPU Lucario totally wrecked me once with four or five consecutive fairs, and I managed to chaingrab the forward-B a couple of times when I was experiementing with him, for example. But I'm definitely not seeing the "everyone's combo ability is so much better" thing you people rave about at all the time.
5) The gimping is out of control. I played for two hours and saw maybe two or three non-gimp KOs. Screw spiking...pretty much any aerial landed at any time off the stage seems to be a guaranteed KO. R.O.B. can kill pretty much anyone by tossing them off and doing one fair, and there's nothing they can do about it. Percents barely even seem to matter anymore.
Honestly, I think the main reason a lot of people find Heavy Brawl appealing is the illusion created by number 4. It creates the impression that Brawl is more like Melee, but it seems pretty superficial if you ask me. Yeah, people come down faster, but by the time you're in range of each other, the hitstun is over and you're in the exact same position you would have been in regular Brawl. You might hit them again and keep them in the air, or they might jump away or airdodge through you. Actually, in theory, the second option should be even easier in Heavy. It makes it possible to combo with a few very specific aerials for a few very specific characters, but Link or Bowser or Ganondorf don't magically get combos.
Say I'm playing as R.O.B. in regular Brawl. As R.O.B., I have a pretty sweet defensive game on the ground, so I'm down there messing with my opponent when they finally manage to get a grab on me and throw me upwards. This is bad, because R.O.B. is pretty much defenseless in the air when someone is underneath him. I try to knock them away with dairs and nairs but they're too slow, and I keep getting hit upwards over and over again. I try to airdodge through them to the ground, but I'm so floaty that I still can't make it. This goes on for a while, and then I finally reach the ground after taking a large amount of damage. And since it takes so long for me to fall, this whole exchange takes, I don't know, 20 seconds.
Now let's look at this scenario in Heavy Brawl. I still have barely any shieldstun and my downsmash is still ridiculously good, so I'm still playing defensively and doing pretty well for myself. Then the grab lands and I'm up in the air. Sure, I fall faster, but the hitstun is the same. By the time my opponent and I are in range of each other, we both have the exact same options we did before. So I get hit by the same attacks for the same reasons, probably about the same number of times, and I hit the ground after taking about the same amount of damage. But this time the whole mess only took eight seconds. That's kind of nice, and a bit more fun to watch, but so what? What really changed to a meaningful degree? It LOOKS a bit more like a Melee combo, but it's just an illusion. My opponent doesn't actually have any more combo ability in this situation. It just looks like they do.
I don't see how this is supposed to magically make the game less defensive, either. That's caused mostly by the lack of shield stun, which Heavy Brawl does exactly nothing about. I mean, even if combos DID benefit, what would that solve? What's keeping the camper from shieldgrabbing you into a downthrow and then comboing YOU? In fact, I'd go so far to say that Heavy Brawl, if allowed to develop, would eventually start to promote camping. I mean, I COULD attack you, fight you, build up your percentage to 120 or so, and then KO you out the boundaries of the stage. Or I could stand on the edge, spam projectiles, and if you come over here, I'll shieldgrab you, toss you off the stage, and kill you at like 15% with an unavoidable aerial (since if you attack or dodge, you'll be dead before the animation ends anyway). It's almost as bad as stages with walk-off edges when it comes to camping.
Regular Brawl's edge game works the way it does largely for the sake of character balance, I think. When a fair percentage of KOs reach the stage boundaries, that means things like power and weight MATTER. Bowser will live considerably longer than Sheik, and Bowser will kill considerably sooner than Sheik. He relies entirely on those attributes to function. In Heavy Brawl (much like in Melee), these things don't matter. Bowser's size and strength don't matter when anyone can tap him with an aerial at 40% off the stage and laugh as he falls to his death. The more punishable points there are in his attacks, the more opportunities there are to knock him just past the edge of the stage, where he can be killed at the other player's leisure. And what benefit does he get in return? What use is being able to kill people at 115 instead of 130 when a faster, more maneuverable character can easily start gimping anyone before they reach 50%? We'll be back in Melee's balance arena, where speed is great and power and weight are pretty much worthless by comparison. Except a bunch of new characters will probably reach top tier because they can gimp at zero.
In a nutshell: Heavy Brawl doesn't magically fix camping, the over-reliance on gimp kills has the potential to really mess up the game (especially since you know that the developers didn't even think about balancing this mode in the slightest), and the few benefits it does have don't seem tangible enough to merit destroying a bunch of characters.