Haruhi isn't God, and she isn't the potential for evolution or whatever. Honestly, I don't know what she is. She's more than a classmate or an ordinary person, that much is for sure, but she's human.
All I know is that she definitely has a power that allows her to change the world around her. If she asks Mikuru to start shooting miniature black holes from her eyes, than you'd better hope Mikuru isn't looking at you. If Haruhi wishes that cats could talk, you're about to have a philosophical discussion with a calico. If Haruhi wants to win a baseball game, than you'd better make sure she wins or there will be a closed-space to deal with soon enough.
I know this is hard to believe, I thought it was a joke at first myself, but I have witnessed too much proof to deny it. I've entered Koizumi's closed-spaces with him and seen his celestials. I've seen Nagato battle and destroy the other alien Asakura. I've even traveled three years in the past with Mikuru. I have no doubt that these people are who they say they are, no matter how insane it sounds.
All I know is I wish Haruhi wasn't so aggressive. I don't mind tagging along with the Brigade occasionally (I admit that sometimes I enjoy seeing those four's powers in action), but how did I ever get so involved in all this? Whenever Haruhi comes up with a new insane idea, like blackmailing the computer club or entering a baseball tournament, Nagato doesn't say anything, Mikuru is too shy to stand up to her, and Koizumi just goes along with her, leaving me to take care of the situation. Just leave me alone, Haruhi. I don't want any part in your outrageous schemes. I never asked to join your ridiculous Brigade.