Martina - unfortunate that she got ill, although she didn't seem particularly excited to be there - could have been good casting, they were just unlucky with circumstance
Richard - like Anthony Ogogo, was too injured to compete, it was just less visible. He was pretty bitter when he left as well - not good casting
Rav/Lynda - didn't gel with their partners at all - might have been good casting but poor work by the sorting hat
Joe - good casting just because the gap between the hype and the reality was the most hilarious thing to happen all series, also Kristina got a shag out of it good for her
Jo - good casting because she took the edge off Brenda after he had to play the villain for five series on the trot
Zoe - potentially good casting but was stuck with James when he couldn't catch a break with the public vote, so poor pairing-up work because he really could have done with a year off ringers.
Craig - didn't gel with Flavia, and created a sour taste generally when the judges called him delusional but kept on saving him in the dance-off, only encouraging him. Doubt he ever could have been good casting though
Phil/Jade - likable journey contestants who both got scuppered by injuries - could have been good casting but again, unlucky with circumstance
Ricky G/Natalie - eh - both overmarked and annoying, don't think either were good casting but I can't really judge objectively because personally they irritated me
Laila - just an all-round car-crash. Didn't gel with her partner, picked up injuries, got involved in scandals, didn't have much time to train. Somewhere between being poor casting and being alright casting done in by circumstance
Ali - feh
Ricky - could have been good casting but like half the others done in by circumstance (getting arrested, dumping his girlfriend mid-run, Natalie not really "getting" the show, even if they did get on)
Chris - saved the show for the mainstream audience, so good casting from that angle, but it has to have been an accident because he's never showed anything like the charisma people saw on the show before or since.
So I think it was a mix of poor casting, poor pairing, and bad circumstance. It could have been a decent cast, especially if it were a 14 person series, and God didn't so relentlessly the show with bad luck.