Ripoffs, similarities, or something else?

It no secret that music "borrows" from the past. (If you believe this series of videos every creative work builds on something that came before.)

My question is, when does something cross the line? When does it become less borrowing and more theft? Is it in intent, in acknowledgment, or other grey area that's hard to pin down?

I think that's a little bit of what the site Sounds Just Like is trying to pin down. It's not as much a condemnation of the artists as it is a place for people to point things out. What you read into the similarities is the only baggage there. 

It is interesting to note how the comments change based on the perceived artistry of the artist. If the commenters generally like the band, it's billed as homage or inspiration. If the band/artist is less admired (typically pop stars from the past 5 years) it's a rip.

No real conclusions here. Just something I've been thinking about.