Hello forge.
I'm a 3rd ed. DnD guy.
This is my first post.  I'm designing a breakoff of the d20 system called the Adapatable RPG.  Major difference: No class.

I figured that there are only 4 serious class/proffession catagories:
major combatant (ranged or CC)
moderate combatant/expert (has more non-combat abilities)
moderate combatant/minor spell-caster (has a limeted amount of spells)
major spellcaster (relies totally upon spells)

In ARPG 1st lvl characters begin with 1000XP with which to buy traits (like feats) or skill points.  It is assumed that they are a major combatant unless they choose to buy (for free) one of three traits that would change their "class"
Any specialization beyond these 4 catagories would be determined by how you navigate through the trait trees allowed to your character.

Is this plausible?  Below are the 3 "class-determining" traits.
Hit Points: At first level you have a number of hit points equal to 10 plus your body modifier.  And each time you level up you get additional HP equal to 1d10 plus your body modifier.
Base Attack Bonus: At first level you have a base attack bonus (BAB) of +1.  Each time you level up this goes up by 1.
Magic Points: You have zero magic points (MP) unless you buy a spell-caster trait.
Spells: This is a special ability that requires MP to use and can only be bought by characters with a spell-caster trait.
Specialist Traits: This is a special type of trait that can only be bought by characters with the expert trait.
If you don’t decide to be a typical fighting adventurer and want to have some other sort or power and skill besides combat there are 3 different traits that you can buy at first level that will dramatically change the options that your character has.  These three traits are described below.
EXPERT: This trait costs 10XP and allows your character to buy up to three specialist traits times your level.  It also reduces your HP at first level to 8 plus your body modifier and when you level up you get additional HP equal to 1d8 plus your body modifier instead of 1d10 plus your body modifier.  Also, instead of beginning with a BAB of +1 you start with a BAB of +0.  When you level up you only increase your BAB three out of every four levels so that at 20th level you have a BAB of 15 instead of the normal character that would have +20 at 20th level.  The idea behind this trait is that you give up your toughness and combat ability in order to access cheap traits that are cunning and usually intelligence based.
MINOR SPELL-CASTING:  This trait costs 10XP and allows you to buy up to three spells times your level.  Like the expert trait you reduce your HP at first level to 8 plus your body modifier and when you level up you get additional HP equal to 1d8 plus your body modifier instead of 1d10 plus your body modifier.  Also, instead of beginning with a BAB of +1 you start with a BAB of +0.  When you level up you only increase your BAB three out of every four levels so that at 20th level you have a BAB of 15 instead of the normal character that would have +20 at 20th level.  When you take this trait you get 50 magic points (MP) plus 10 times your soul modifier.
MAJOR SPELL-CASTING: This trait costs 20XP and allows you to buy a number of spells equal to 2 plus your soul modifier times your level.  This trait severely reduces your HP.  At first level you have 6 plus your body modifier instead of 10 plus your body modifier.  And when you level up you get 1d6 + your body modifier as additional health instead of 1d10 plus your body modifier.  In addition to reduced health you also start with a BAB of +0 that only goes up one out of two levels so that at 20th level you only have a BAB of +10 instead of +20.  The upside to this skill is being able to buy spells and having a lot of magic points (MP).  When you take this skill you are granted 100 MP plus 10 times your soul modifier.

Also should specialist traits and spell be awarded insted of bought to make up for the loss of HP and BAB?

thanks --aaron

It doesn't look to me like it really gets rid of class (though you are decreasing the number of classes to 4), now you just have becoming a wizard/sorcerer etc.. cost XP, and you aren't calling it the same thing, but you still have what type of character someone is linked to their BAB, and hit points.  If you really want to get rid of class let people buy their BAB, their spell-casting abilities, and their hit points separately.  You still end up getting something that resembles the classes in D&D potentially (a person could buy the BAB, hit points, and spell-casting abilities that a Sorcerer would have) but at least that way it's not like you are just getting pre-packaged classes in a different way than you would in standard DnD.  That and having to buy the ability to even use spell-casting abilities, along with lower HP, and weaker BAB pretty much amounts to giving a sorcerer/wizard type an XP penalty, are they really powerful enough to warrant that?  You do give them better hit die than they get in DnD (well better than arcane spell casters do anyhow) so an XP penalty for them might be ok. 
Anyways if I were making a classless D20 system (and I sort of am, but it's very different than what you're trying to accomplish) I would get rid of linking spells/expert abilities or whatever to HP and BAB, and just have all those things bought separately.  I would just make spell-casting etc... cost enough XP that there's no need to take an additional penalty to your BAB and HP.

Anyways you might want to actually post this on Wizards forums, and get some input from people whom are really into D&D and making D20 house rules (they have a whole forum for house rules I'm almost certain).

For extra inspiration, you might have a look at Anime d20 which takes d20 and turns it into a point-based system.

Class-lite 1d20 system: True20 / Blue Rose

Classless 2d10 system: EpicRpg.com

I'm pretty sure the mages on the water put out an official class-lite system?