I agree with Centre of All here - but see where Hyena is coming from...
But ultimately, I think that belief and theme is one of the key things that makes Planescape interesting, and if it's just a plane full of fire and people who happen to live there ... eh.
One unified theme for each inner plane would suck, but having a look at what people believe about (say) fire and working that into the plane is a good idea. There are reasons why things like purification and destruction are associated with fire.
The Outer Planes exist because people believe in those concepts, and thus closely reflect them.
The Inner Planes exist because they're fundamental - but the concepts which people associate with them are already there. They're not as dominant as with the Outer Planes because ultimately they're still about their element rather than a theme - but the themes that people see in fire are real things that would exist in a plane that is fundamentally about fire.
So, if there's a culture that associates purification with fire - chances are that there's going to be at least some aspect of purification in Fire somewhere - it's part and parcel of the whole 'fire' deal - they're getting it from somewhere, rather than the abtract.
That's my justification, anyway. Even without a justification, it's probably still a good idea to give the inner planes character - our group used to play an episodic game with a new Plane each session ... yeah, the inner planes were never picked. They're just not as captivating as planes which personify a belief - it's just a ... whole lot of fire.
Edit:
I guess what I'm saying more succinctly is: The Plane of Fire should be about fire and everything associated with it - rather than just a plane where everything is on fire.
""Simply put, making the Inner Planes just "raw, untamed, undiluted" elements makes them boring. Without the character, there's little reason for there to be multiple Inner Planes.""
To me, it is the individual inhabitants and their environment that give the plane personality, rather than the plane itself. It all depends on the circumstances and races of the beings that live there.
""certain planes tend toward certain alignments," you are already attributing belief to the plane in the same way I am. ""
Yes, I agree, but it stops beyond the "true neutral" boundary. To me, beyond that, it varies by race, individual, and environment. Like I said, the Quasiplane of Mineral is a place of extreme xenophobia. Not because Mineral embodies xenophobia, but due to circumstances. They have good reason to hold any non-native with great suspicion. I can't picture them not being unfriendly and xenophobic to outsiders as a whole. Sure, there is the concept of trade, but really, what would crystalline beings and armored outsiders who eat minerals want that isn't already on Mineral?
""I'm just taking it an extra step to show that the elements can and should represent multiple beliefs. The beauty of neutrality isn't in its complete apathy, but rather in its willingness to accept and become anything.""
That's exactly why I am opposed to giving it specific arbitrary traits (e.g. the people of fire are passionate and emotional, the people of ice are apathetic, etc.) The Elemental Planes can embody anything, just as you said-- it varies by species, environment, etc. By assigning specific personality traits to the entire plane, you limit what it can be. I admit I am doing that to a degree with mineral, but I just can't fathom it being (from a logical standpoint) any other way. I mean you're talking about a plane where outsiders come all the time and steal from your land.
""I don't think Pathguy is on the right track all the time. Ice can be beautiful as well as dreary, delicate as well as sharp. Don't all four of those aspects deserve to be a part of Ice's character?""
Yes. However, I would not want to limit it to just those concepts.
""Does that help put it in a better perspective? I'm not trying to say the Inner Planes should be formed by belief itself, but I think that removing abstract concepts and beliefs entirely from the Inner Planes is shortsighted and ultimately dull.""
Somewhat. I still disagree that it becomes dull without abstract concepts. Mainly because, once again, unlike the Outer Planes where both the plane and the people are defined by abstract concepts, I view the people of the Inner Planes alone as defining the abstract concepts, only to a much greater variety. Different towns and different races would symbolize different abstract aspects of the element, and this goes on to detail some of the conflict on these planes.