Danny was just...bewildering. I liked his first episode, but after that, could they have picked a worse boyfriend for Clara? Jenna Coleman had genuine chemistry with Matt Smith. They were magical. She also has some chemistry with Capaldi, but the writers have limited the full potential of that, stupidly IMO, for story reasons.
She had zero chemistry with Samuel Anderson. Their characters' "relationship" was just weird, and as such, Danny's character arc had no impact with me.
And Danny was just weird, as well. They could've had Clara tell him about how the Doctor was a soldier himself and fought in the Time War (and bucked authority) and Trenzalore, or they could've had the Doctor tell Danny himself, to shut Danny's "Yes SIR! Officer, SIR! Don't get your hands dirty, SIR!" crap up. 30 seconds of screentime is all it would've taken.
Or just....take a trip on the TARDIS with your girlfriend and see why she likes it so much? Or instead of seeing the Doctor as controlling Clara and getting her to do his bidding, maybe see that by traveling with him, Clara herself gains confidence and self-expression, doing things she didn't think she could do? "Flatline" was a perfect time for that. Danny and the Doctor stuck on the TARDIS together while Clara does her thing. Danny and the Doctor could've had it out and Danny could've observed his girlfriend be awesome. But what did we get instead? Him saying "Whaddup bitch?" over the phone. (Yes, I know he really said "I've got our bench")
I don't know...he was just a poorly-executed character. But the worst thing is, because of him we'll never really see how Twelve and Clara could've really operated as a team, since the Danny storyline was shoehorned in to provide "conflict" and doubt between them. And now Jenna is leaving.
That said, Danny was still better than one more second of River Song......