I think I just pooped a little.
I want to make hawt, sweet, inappropriate love to Microlite20.
Seriously, this system (I skipped all 40 pages, so I'm talking about the core PDF you handed out) makes me want to DM again.
What I love about it is how easy it is to ad-hoc and customize the content.
PC 1: I want to play a Drow Rogue! What are my stats?
Me: Hmm...Drow...Dex +1 Mind +1
PC 2: I want to play a Half Orc Warrior!
Me: +3 STR -1 Mind
PC 3: I want a Half-Celestial Cleric!
Me: +1 Mind +1 Communication +1 Social, but you're 1 level behind.
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The ECL leveling system is genius.
The encounter system is brilliant.
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The only modifications I would make would be the following:
Warriors get +1 BAB every even level (in addition to +1 per level), Rogues are +1 per level, and Mages/Clerics are +1 every even level (no +1 per level).
Warriors are All Armor/Weapons, Rogues are all One Handed weapons and ranged, Mages/Clerics are all 1d6 or lower weapons and ranged (Blunt for Clerics...I'm old school! Leave me alone!)
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Spells are even simpler than you say. They still cost HP, but I'd do something like:
Mage:
-Damage
-Control
-Charm
-Summon
Cleric:
-Heal
-Buff
-Damage
-Protection
I would then come up with a constant variable (1d6 per level) and let the PCs decide what level they want to make the spell. Something like Die Number/2 = Level (round up)
PC:I want to cast a 5d6 Fireball!
Me: Ok, take 3 HP damage!
PC: I want to charm an NPC with a DC of 18!
Me: Ok, take 4 HP
Or something like that.
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I love it! I can't say that enough!
Thanks for putting it out there!