Well, I will avoid posting critique on your page design.
There will be no graveyard?
Any and all of a players... "prepared spells" will be accessible to him, assuming he has the mana to cast them?
This brings into consideration how powerful combo's will work.
Even with you re-evaluating everything's "mana cost" (which is a bad idea IMO), there will still be powerful combinations with several "spells" that, on their own, are not powerful. One see's these combo's in the MtG championships. How will you add mitigating factors to this?
A few cards I am curious about:
Slivers (all of them, and their unmatched synergy)
Non-lands that generate mana.
Special lands like Cloudpost.
Cards that give/remove turns.
Cards that allow one to "Put things into play" without casting said things as spells (or paying their mana costs)
Cards that interact with the graveyard system.
There will be a "graveyard" that in game terms will be called the "mana trail" its a echo of you will of all the spells the caster has used. This will allow such spells as antimate dead, spells with Flash Back and a few other key words to function properly.
Yes, a caster will have access to all prepared spells, but there will be a limitation on how many spells he can prepare.
One thing that you must remember about combos (especially those with creatures as part of the combo) it will be much easier to kill creatures off. One cannot simple cast a creature spell and not block with it as in regular magic. If a creature is on the battlefield it can much easier be dispatched.
Slivers are going to be powerful, but as with regular magic multicolor decks are going to be slow. Why? Lets say you are a red/black mage. During your mana phase you roll your D8 mana for the turn. You roll a 3. Since red is your primary you will only recieve 2 red mana and 1 black one. If you are a red/blue/black/green mage and you roll a 3 you will only get 1 red, 1 blue, and 1 black with no green. So while multi color mages will have access to more spells, their mana is going to be severly limited.
The Tap keyword has been changed to "exhaust" So say you are a green mage and you cast a llanowar elves. On your next turn you may exhaust him to give yourself 1 additonal mana (remember you may NEVER under ANY circumstances go above your maximum allowed mana)
Land cards as a whole are no longer used. Even non-basic, artifact and any other kind of land. They are NOT used.
If a card would allow you to take an additonal turn, like time walk you will take an additonal turn at the end of that turn (IE after everyone else has acted for the turn, if for some reason multiple players get an additonal turn then they will take the additonal turns based on initative) And conversely, if you were to loose your turn then that player will skip his or her NEXT turn.
I am not saying that it is impossible to break the system. but there are TONS of broken things in D&D, (pun pun anyone?) If you are LOOKING to break the system I am sure you can. But that isnt the point. People will be able to use combos, and that is fine if they do. Also remember this isnt just a duel application for two people to sit around and go at it, if you wanted to do that play a normal game of Magic. This is ment to be an RPG, with multiple players, a DM, and powerful monsters with their own counter abilites and tricks up their sleeve.