you obviously played d3 on a old pc ^^ it's FAr from boring. If you think being scared shitless boring i give up ^__^
Doom3 is designed for certain gamers, it wasnt made to be a complex story with tricky puzzels to solve, that's the biggest mistake when people say it's just a mindless shooter.
Where as HL2 have the freedom of pritty much what ever they want to do, which means it will apeal to a larger player base. I think Doom3 was a work of art plus it feels so damn polished. I think it was the perfect balance of story + objective + blowing the hell out of amazing animated enemys.
It's like comparing Serious Sam to Halflife, saying one is boring because it's more like a deathmatch is silly, you shouldnt have played the game if you don't like that "sub genre" of FPS's but im guessing you downloaded it anway.