Bad Wolf. It really doesn't mean anything, the actual phrase "Bad Wolf". It sounds cool and mysterious, but there's really no context on why it's called that. It might as well have been Rubbery Penguin.
It's not even a really important phrase. While I'm sure the internet was ablaze with theories at the time, until "Boomtown" there wasn't any evidence the characters noticed it, and when it is, it's just demised with a hand wave. Until the last few minutes of the series, it has no real importance on anything. You can watch all of Series 1, edit out Bad Wolf, and nothing would change.
The most disappointing thing about it though, it doesn't have any actual affect on Rose's character. When she sees it in "The Parting of Ways", it's treated like she's had some sort of revelation, but there is no actual revelation. Rose never doubted she would get back to the Doctor, she knew it as a fact that she would do what she could, she just didn't know how. Seeing "Bad Wolf" doesn't really change anything for her. It didn't even tell her that opening up the TARDIS would help.
If Rose had given up, if she had lost faith in herself, that would have been better. It would have been great to see her being built up from such a low, but since she already starts out on such a high, there's nowhere for her to really go.