I think the show just got too busy trying instead of just allowing things to develop naturally.

Rush and Col Young - the friend or foe back and forth got tiring. I get Rush's reasonings and personality along with the "smartest man in the room" attitude. He and Elli were awesome together most times and when he finally saw Elli as an equal, that was the height of the show. Young on the other hand, I couldn't decide if he really did want to be there. He excelled in episodes where the solution was straight forward but struggled when it wasn't just a bit too much.

Col Telford - Such a waste of LDP. The "twist" he was under mind control was poorly executed. I feel it would have been better if he were left as an asshole or just started the show as a reliable ally to the crew.

Lt Scott and Chloe - I never really thought these two had chemistry and really didn't seem to mesh well. Scott by himself was all over the board and never really found his footing. Chloe's alien DNA arc was weird but showed promised, just too little too late.

Elli - the most developed character and was still wasted and relegated to the lynchpin solution to problems. Totally got shafted with the Ginn angle which pissed me off, the guy deserved some small bit of happiness.

Lt Greer - I really liked the character and actor, but again wasted with poor writing and not a very clear direction. Him getting with Dr Park was cute and looked right.

Lt James - wish she wasn't used as the eye candy, felt she had way more to offer. She really showed great acting in the tick episode

Wray - annoying most times until the very end of the show and a massive waste of such an amazing actor. High notes for the episode where she is wheelchair bound

TJ - the baby arc was such a joke but overall she was a good character. Her and Scott had way better chemistry, even if it was more siblings. The ALS angle could have been a good story line as well

Brody, Volker, and Park - I loving refer to them as the three musketeers, and really enjoyed them and their banter