Admirably simple resolution mechanics. The only suggestions I could call to mind would be to provide renamed abilities in some cases;

[list]
[*] [b]DeProtect[/b] --> [b]Sunder[/b] (removes Protect defenses, increases physical damage taken).
[*] [b]DeShell[/b] --> [b]Shatter[/b] (removes Shell defenses, increases magical damage taken).
[*] [b]Imperil[/b] --> [b]Vulnerability (Specified Element)[/b] (specifies which increased elemental type of damage taken).
[*] [b]Zombie[/b] --> [b]Inversion[/b] (swaps all vulnerabilities and resistances, which could work for better or worse).
[/list]

In addition, some possible other (rare and powerful) conditions to consider.

[list]
[*] [b]Dazed[/b] (cannot take magical offensive or defensive actions).
[*] [b]Stunned[/b] (cannot take physical offensive or defensive actions).
[/list]

Short of that, very simple and effective in the hands of fair and well mannered users.