First post in this forum! So before I respond to the post, I'd just like to say, Great game, Kornel.
On the topic of levels in RPG's, they are not strictly needed, but every game has some sort of leveling process. Whether it is D&D, or Cyberpunk 2020 (no levels, only skill points earnings) Really, the point of an RPG is to have some sort of leveling system, or else it isn't an RPG, it is simply a roleplay. Whether or not those levels have a major impact on the game is another beast entirely (Cyberpunk 2020 vs. D&D, no matter how much you 'level' in Cyberpunk, which means increasing your skills, you are still easily killable. D&D is the opposite).
Honestly, I would have to say more RPG's do have levels. As long as the system is balanced, even god-like powers aren't bad. Anyways, you can just choose what you prefer.
One last thing, if you think leveling systems in RPG's are bad, how would you propose changing it? Besides, it seems to me the leveling system in DoomRL isn't strictly a leveling system. Its just a good way to fill in the process of how your Doomguy picks up tricks and tactics as the game goes on, instead of getting simply "stronger".
Meh, sorry if i rambled a little bit off that, I just hate leaving thoughts incomplete =)
Great game again Kornel!