9. River Song
River Song is almost as divisive as Steven Moffat himself. To some fans, River is a fantastic new take on the companion story with a uniquely complex relationship (both temporally and emotionally) with the Doctor. To others, she's the epitome of some nebulous criticism of Moffat's inability to write strong female characters without resorting to a hypersexualised action-girl trope with a downright dependence on her man... or rather, Time Lord. Technically, the concept of meeting a character out of sequence has been done before. The Sixth Doctor met his second companion Mel during his trial, by which point she was already familiar with him though he had no idea who she was. In the Big Finish audios, companion Charley Pollard (no relation to Vicky) originally travelled with the Eighth Doctor only to later partner up with the Sixth Doctor as well. Though Mel's origins weren't explored on screen, her time as a companion lent her tenure a certain sense of continuity that River's sporadic interactions with the Time Lord lacked. Though the idea wasn't entirely original, and whether or not you like River as a character, you have to give Moffat credit for his ambitious approach to developing a time travelling ally for the Doctor over the course of several years. Whereas Davies took the show one year at a time, having taken a risk by helming the relaunch in the first place, keeping each storyline and character arc fairly self contained in case the show got axed, Moffat took advantage of the show being established to experiment with storytelling. How well it worked is a matter for debate, but you must credit Moffat having the brass to try.