The sad thing about this thread is that the only posts that are convincing are the ones saying this stage is a good stage...
As much as some people want to stick their heads in the sand and pretend it's not true, a major part of Brawl's design is that every stage is substantially different from the others. The fundamental way you approach gameplay has to be deeply informed by what stage you are on. Let's take the match-up Mr. Game & Watch vs Ice Climbers:
On Battlefield I'm going to focus on mobility between the three platforms when I have a lead as G&W. ICs basically have to run after me with uairs and Blizzards, but they have to be really careful since I can reverse direction and nail them with an aerial if they get sloppy. If ICs have a lead, they're going to just pick a side platform to sit under and uair if I get on it, making them very hard to approach.
On Smashville I'm going to be riding the moving platform around a lot. ICs are going to wait for me to pass over them and try to snipe with uair. I'm going to try to use Fire to punish sloppy uairs on shield to pop them up into position for a nair. If they get a lead, I more or less have to give up the platform play entirely which makes it extremely hard to fight them.
On Rainbow Cruise I'm going to focus on mobility and evasiveness on the ship and try to tag them with aerials if they try to chase. I may try to take a defensive position on the back of the boat and try to parry approaches. When the ship moves to the carpet section, I'm going to try to stay low and see if I can really get at them with nairs. If I can force them into real danger, I'll try to start chasing from the right to see if I can get an early fair kill which can really seal the deal for me. On the third section, I'm going to definitely focus on keeping to the right of ICs, staying on the platforms a lot, and seeing if I can snipe at them with fairs that will be hard to respond to. It's far in my interest to create trap situations when the stage is about to move downward since they have twice as many things that could go wrong as I do, and even if I don't gimp outright, I can land some good hits in the air as they come down under pressure. ICs, on the other hand, are going to try to corner me on the ship, try to just avoid combat completely on the carpets since that section is so wildly bad for them, and try to stay close to me during the third section since that's a pretty good time to cg me.
On Frigate Orpheon I'm going to stay to the right of the stage on the first form and try to "hold" the seam between the main stage and the part that moves. The vertical differences that frequently arise here make it much harder than usual for ICs to move in, and if I get in a situation that's too dangerous, I can use Fire and then drift to the left platform. On the second form, I'm going to hang in the center and try to counter approaches as best I can; it's a hard position to approach if played right. If ICs take the lead and start holding positions far from the good positions for G&W on this stage, I'm going to try to get on the ledges and fight back from those. If ICs move away from the effective attack range of ledge play, they move into areas where G&W has a lot of strong counter-play with geography on both forms.
Yeah, there are a lot of common elements, but it doesn't change that from the start to finish everything I do in the match is focused on the geography. I'm trying to create advantageous positions and avoid disadvantageous ones. I may very well be trying to run the clock and looking for ways to make hitting me hard either through stationary defense or by mobility based running. Geography dictates what positions are advantageous and disadvantageous, and it also dictates what the options are to hold defenses or approach a defense. The natural design of stages in Brawl only serves to make this more prevalent as only three of them are non-interactive (all three of which are geographically strongly different from each other) and many stages have completely unique elements to them relative to all the other stages (stuff like Distant Planet's rain effect that just has no analogue on any other stage). The amount of competitive depth this adds to Brawl is insane. Brawl already had 666 character-character match-ups. Using a different stage in many ways makes it a new match-up so if you were to include all 42 stages uniquely you would be looking at a game with 27972 match-ups. If more people looked at banning a stage as banning 666 match-ups, maybe things would be looked at differently... No one would argue stuff like how X stage changes the game too much; that's just not a reasonable position. Changes it from what exactly? You can't find me two stages that are the same or even really all that similar in the first place so a stage being an "outlier" should hardly bother us.
Likewise, the fact that that stage so deeply informs our game plans always makes the idea of "fighting the stage" or "not focusing on your opponent" so silly. You're focusing on defeating your opponent always on any stage. It's not even theoretically possible to make a stage in which you don't focus on that. You make use of the stage, on every stage, to create positions of strength for yourself. If the presence of a stage hazard is part of what makes your position strong, what difference is it?
This stage isn't really very random at all. This has been well established. It has randomness, but it's within tightly bound parameters that allow any player with the minimal skill needed to cover multiple possibilities with single actions to handle it easily. Anyone arguing anything based on randomness is being kinda silly here. It is hazardous. It has predictable hazards. The odd equation of hazards to randomness is perhaps tripping some people up, but there's no logical correlation between random factors in matches and stage hazards. The mobility argument is similar. How does being faster help you if both sides know where you need to be well in advance? The argument literally doesn't make sense.
There are a few real issues here, but I don't think a single person has brought them up (the only "real" issue brought up is the appeal to numbers of how no one likes the stage, which is definitely true but really speaks badly of the community more than wins any argument over what is competitively sound). I see four real issues:
1. Mario Bros.' level of hazards combined with the teching prominence cause wild fluctuation in stock length, including the possibility of losing stocks super fast. You might make a mistake that causes you to die at 50% to a thrown item. The difference in play that causes that and that causes you to live to 200% is very small. This makes matches between similarly but not equally skilled players basically completely unpredictable in terms of result.
2. This is somewhat related to the previous point, but the ability to tech (which is continuous, not boolean) is tested more strongly here than on any other stage by a big margin (even stages like Luigi's Mansion focus on teching far less than this stage). This causes discrepencies in teching ability, an ability that is mostly tech skill, to be far more pronounced in match results. Game elements that cause tech skill to be significantly more pronounced in match results are really not good for the game, though the only reason people would agree with that is because I'm saying it about Mario Bros..
3. The unique attributes of the charcters are somewhat understated on Mario Bros.. Yeah Brawl has those 27972 match-ups, but a lot of them are very similar. Fox/Bowser/Temple and Fox/Ganon/Temple are essentially the same match-up, and anyone with a basic understanding of the game could understand why. Mario Bros. pushes this even more by making wide numbers of the character attributes just not that significant. For instance, G&W's usmash on most stages is an attribute that helps define what it means to play as him and against him. It's essentially not a move on Mario Bros.. In fact, G&W's only ground moves he should ever use on Mario Bros. are dsmash (near walk-offs in certain ambiguous situations), dtilt, a few other walk-off pressure situations, to flip hazards), and jab (exclusively to flip hazards). Most characters get similarly boiled down so the 666 match-ups that exist on Mario Bros., playable though they may be, represent a far less diverse and interesting slice of gameplay than the 666 match-ups belonging to really any other of the non-loop stages.
4. The nature of the gameplay on Mario Bros., partially because of the low lack of variety between match-ups, most likely makes the skill ceiling on the stage low enough to be realistically human achievable. That is, beyond a certain level of skill on Mario Bros., all players are likely essentially equal. That's not good for the game to have elements like that. BPC did bring up "mitigation of skill", but I don't think the Mario Bros. metagame is advanced enough for this to be a currently present factor so much as a very likely future factor so only partial credit for that one.
Of course, the stage hasn't been carefully tested, and it may have surprises in store. There's definitely a lot of reason to be suspicious, and when most people are ready to throw away proven quality stages like Norfair, I'm disinclined to be pushing for such a massive experiment with many ways to prove itelf a problem like Mario Bros.. It's just unfortunate that the arguments within this topic failed to be compelling, but then again, I think most people base their stage preferences more on, well preference and gut feelings than on analysis.