What I find makes Vel'koz special is that he most versatile attack options of any mage I know of. I feel like people categorize him as an artillery mage without knowing how many offensive options he has. Its not wrong, but he can do more. Champs like Xerath or Lux basically use only poke and burst. But for Vel'koz,
Q - poke/slow
Q > W - Kite combo. Damages and slows in a line while moving.
W -> W - instant waveclear
E - Peel
E -> W - AoE. The overlapping area of these AoEs takes a lot of damage.
R - DPS/Snipe. Even tanks can be melted if they try to stand in his ult. But he can finish off kills with the tip of it as well, because it reaches its long range instantly.
EWQR - burst. Alpha Striking someone with all his skills usually causes them to explode.
I don't know of another champ that can attack in so many ways.