A couple of suggestions, speaking as a front-end designer / occasional indie-game artist:
- I don't know how you're calculating it, but consider adding a bit of randomness to the effect radius. This way, you won't be blasting perfect circles each time.
- If keeping the graphics CSS-only isn't a priority, consider some sprites! If you need a sooty blast scar or maybe a simple explosion sprite, I'd be glad to lend a hand.
Some randomness would effectively be great to add! As for the sprites, I really wanted to focus on font and simplicity. It is much more about the chaos of many destructions, than about a single explosion.
I might add different velocity depending on the mousedown event duration. The longer you press, the bigger the explosion get.
When I looked at your JS, I thought to myself "that is a reasonable amount of code for that kind of functionality...even a little less than I would've thought"...considering there's no dependencies to browser-taming libraries:
https://github.com/plehoux/fontBomb/blob/master/js/particle....
Source is here: https://github.com/plehoux/fontBomb/tree/master/src/coffee