To be honest, the idea of being sent back in time is what makes it scary for me. SCP-173 is just your average run-of-the-mill "ded. nawt beeg surprais" type monster.
Not to mention the eternal loving flamewar between the Doctor Who and SCP fans over which blink-and-die monster came first - which is actually how I found out about and got interested in SCP, since I was a Doctor Who fan first.