My personal list of important features to add

Let abilities apply a status to a token. Have those statuses removed when the effects that apply them are removed.

Let abilities be attached to attacks.

"Add Diced" for everything, not just HP.

Change "Add Diced" to "add effect"?

Remove the separate tabs and just have one area.
Click "Add Diced" and the first drop down will be "Attack, Ability, AC.. etc." Each additional drop down can be grayed out until you pick the one preceding it. Have a drop down for "bonus, mod, penalty", allow people to type in dice expressions and modifiers.
Use regular expression to parse it.
String pattern = "^\s*([+-]?)\s*(\d{1,3})\s*[dD]\s*(\d{1,3})\s*(([+-])\s*(\d+))?\s*$";
String diceExpress "-3d6 +12";
MATCH FORMATION:
1. [0-1] `-`
2. [2-3] `3`
3. [6-7] `6`
4. [8-12] `+12`
5. [8-9] `+`
6. [10-12] `12`

Add non-lethal damage.