There's something about bullets that has always made a game feel more real. Perhaps it's the fact that they're more realistic, or that they look and sound more complex than other weapons. It would be great if a few of the mech units had rotating turrets that fired bullets with tracer fire, and would also light up the firing unit and those nearby to create the essence of a powerful weapon firing real projectiles. This effect is illustrated well in Team Fortress 2 in "The Heavy": It would be even more immersive if the sound of gunshots close up would be high-pitched, as it usually is, and the sound of distant gunfire to be muffled and low-pitch. Here's an example of close-range gunfire sounds and of distant gunfire ambience.
I found that a lot in the old days of playing DoD where the bullet impacts where different from the bullet shots. While the use of such things in FPS doesn't necessarily prove much for an epic RTS, i shall raise homeworld. Looking at a battle happening half way across the map with flashes of light from beam weapons and the dull boom of ships slowy exploding, that was epic because it felt as though these things impacted you even though they were really far away.
Having weapons that can disintegrate everything along their trajectory in one shot can also add to the feel that they are powerful. Um, for example, if anyone remembers dragonball z.