For no good reason at all, I will attempt to rebut the criticism:
1) A lone mild mannered scientist single-handedly invades an alien dimension in a climactic battle to save the world. Then, you are approached by the mysterious "G" man in a seemingly man-made "interdimension" who then informs you that there are yet bigger things at hand and gives you an ultimatum: Join him or perish. Yeah, he didn't get the girl, but what more could there be?
2) Perhaps some parts of the game were "unexplained" in order to add mystery to the story and add to the roleplaying experience of a scientist who found himself in the wrong place at the wrong time. (btw, in the beginning, they weren't trying to open a portal a la Doom, instead the portal was an inadvertent consequence of routine testing)
3) Well, Gordon Freeman doesn't want to end up with a Half-Life...
4) Perhaps it was designed to be a close range weapon. There were many other weapons that had decent range, if I remember correctly.
Anyway, that's what I think of it. Maybe I would agree with you more if I played it now, or perhaps our taste in games is just irreconcilably different. I dunno