Overall, this series had loads of potential but it feels squandered. I'm not gonna point fingers and say who is at fault since the Internet and its main actor have done that already. But rather, this is what happens when you're trying to put out a new show in a vastly crowded playing field. Ask any actor who's worked in television and they will tell you some stories about everyone, from the higher-ups to the lowly writers, trying to keep a lagging TV show afloat.
Personally, I wish Marvel did two things with Agents of SHIELD. First, they should have put more time and effort into developing this show rather than launch it a year after The Avengers came out. I know making a television show is much more fast-pace than making a feature-length movie, but this is one of those times where slow and steady could have benefited this series. Second, it should have gone on Netflix or on a cable channel rather than a major network like ABC. These days, unless you're a sure bet (or one of the many CSI or Law & Order spin-offs), trying to compete with other shows is a difficult task and its compounded even further when you're a new series. At least with cable or online streaming, you're able to find a niche audience that will tune in.
I'm still gonna stick with this show and hope for the best. However, unless they pull off a major turnaround once they come back, I seriously doubt there will be a second season to Agents of SHIELD.