First off, Paladins are my favorite class.
In my opinion, I've always seen Clerics as the divine casters and Paladins as the divine warriors. The fact that clerics have decent hp, decent attack bonus progression, and can wear decent armor and wield decent weapons is there primarily to facilitate play IMO, not to shoehorn all clerics into the "battle-cleric" role.
With the next D&D rolling around, I've heard about this differentiation between priests and clerics as they are traditionally conceived. I'm almost wholly in support of "priests" as more squishy or less martial clerics, but if we are then to assume clerics are divine warriors, what does that make Paladins? Paladins are obviously deriving their powers from some sort of divine power, and they best fit the "warrior" description with their heavy armor, shields, good BAB, good HD, etc. I'm fine with the addition of the "priest" but I'm worried about Paladins having their "holy warrior" status being trampled on by the clerics who by all accounts are already much more powerful due to their casting abilities.
The real problem I have with this exclusive mentality that Paladins are always LG is that it's horrible to parse as a DM running a custom setting with a custom Pantheon. The problem is that you end up with "Paladins" of other deities serving as their holy warriors and crusaders alongside the priests but they have horribly mismatched abilities. I want my paladins of the god of magic to have nifty magic powers or something befitting a crusader for the cause of the god of magic, not to be using a "holy aura" with the ability to "smite evil". To the god of magic, good or evil might be a non-issue.
So what about me and other DM's like me? I love the Paladin as it traditionally exists, but if we create prestige classes for every other crusader of every other pantheon then they're totally unusable for folks like me who use a custom pantheon. Why not allow the Paladin to exist apart from alignment or at least change the Paladin's name to Champion or something and then call LG Champions "Paladins"?
That's why in my system I'm calling Paladins "Crusaders" and are tied to a specific religion (or merely to some good "ideal" - in which case they default to the "LG" deity powers but do not derive their powers from the deity but rather from some platonic notion of goodness). The powers they receive are dictated by their choice in deity - so Crusaders for the god of goodness (or the "ideal" of goodness) look a lot like traditional paladins and Crusaders for evil look a lot like Blackguards but in the end you can have Crusaders for the gods of fate, magic, travel, weather, etc. It's a lot easier to build Paladin "packages" for each deity than it is to build a separate Prestige Class for each deity in a custom pantheon.