For those who would like to respect the wishes of those players who want to convert DBZ, here is my general guide. I am a personal fan of DBZ, but the power levels and apparent destruction unleashed, as well as the way their power levels rise so quickly, puts some GMs into daunting positions. Here are my personal ways to handle these problems.

One way is a simpler way: preserve game balance, old-fashioned. Allow energy blasts, flight, super-speed and so on, but merely weaken the characters and their potential power in scope.

The other way is to use continuities of DBZ and different systems. Watch what I accomplish:

1. It is a simple argument to make that DBZ fighters are horribly inefficient. When they charge, the mere energy they wastefully let escape causes craters to be formed. As powerful and interesting this may seem, it shows that their energies are enough that they are difficult to control. Also watch their energy blasts make a crater and blinding light enough to be seen from miles around and not even scratch the enemy. While often these considerations have to do with power level, we see evidence of the same thing when Piccolo and Android 17 fight. The two are equals, and yet when Piccolo unleashes his Guided Scatter Shot, an attack so powerful that the entire area is lit with a light seen from Master Roshi's, 17 repels it with a barrier.
2. For those continuities with cybernetic systems, watch how much more powerful Frieza becomes when he (she?) gets cybernetic augmentation. An additional 1,000,000 is added to his power level. Even considering how good the augmentation was (due to King Cold's money and power), if mere machine enhances so much, then obviously a DBZ fighter's power is decreased in scale.
3. For Rifts continuities, even attacks from Cell or Broli are quickly rendered fairly unimpressive as compared to the ultimate power demonstrated by some of the super vehicles (railguns that do 1D4x100 M.D. per single shot or particle beams with 3D6x100 M.D. damage).
4. Quite simply, punches and kicks are not likely to break through tank armor. That's a pretty bad argument (why can the Hulk and Superman do it, then?), and it is rendered useless anyways by energy blasts. Energy blasts are quite likely to annihilate tanks. That said, we do note that Cell never tries his luck punching the tanks sent after him, but merely destroys them.
5. Look at the weapons unleashed against Cell! Mere missiles and tank cannons are not likely to kill some of the more powerful PCs in many RPGs. Firebrand in Power Struggle can withstand over 5000 damage in Guardian form, not counting barriers; a tank cannon is not sufficient to kill him either.
6. Final Fantasy games have a similar ridiculous ascension of power, with "bosses" replacing "sagas". End bosses of Final Fantasy games appear capable of destroying areas of universal size!
7. Many blasts and power levels in DBZ require enormous amounts of charging, with the most notorious being the Spirit Bomb. When they do do "pulses" of energy (quite common in World's Strongest), the damage is not nearly as impressive.