A small character palette in the sidebar would go a long way to bringing more Unicode awesomeness to StackExchange. I for one am tired of typing -- in place of an em-dash — and we live in 2010, for balpha's sake.
Here's for example a general purpose palette: (­ is being its usual self)
┌─┬┐ αµ ~∀ ±− ↖↑↗ ≤≥ │┼├┤ βϱ `∃ ×² – ←⇔→ ⇐⇒ └─┴┘ πω ¬∊ ÷³ — ↙↓↘ ⊂⊃
Hovering on each item would show a tooltip with a quick description; clicking on it would insert it in the body. I have no preference on whether clicking should write the character itself or its HTML escape representation.