Do:
Stat XP: As you use a statistic, you eventually gain a minor bonus in that direction. Use the Run option a lot, or dodge bullets plenty, you could get a small bonus in speed/dodge. Use healing items frequently, you could get an extra 1 or 2 max HP. Shoot from up close a lot, you could gain an extra ToDmg; shoot from long distances, an extra ToHit. Melee a lot, you could gain...
Non-Doom special abilities: It's a Rogue-like, but not Sword of Fargoal.
Shades: Look, there's what's left of the character I brought in here 8 guys ago. And he's still a mean nos fo a hctib.
Don't
Real-time: It's a rogue-like. It's close enough. It's supposed to be more thinking.
Use space to open doors and control to fire: It's DoomRL, not Doom.
Facing/directional bonuses: See previous.