My biggest gripe with LEDs is that they have no "fade." They're either on or off. To me, games with a full on conversion look "blinky." Sweeping playfield lighting effects look staggered and gritty.
Products like LED OCD are fantastic, but its limited in use. I'm surprised no one has fitted a small capacitor to the circuitry within the bulb to keep it lit a fraction longer so it can "dim out" (as much as an LED is capable of dimming, at least).