The most pressing issue right off the bat for Planetes is the very nature in which it is told, which is a semi-episodic structure. The first half of the series is heavily episodic, which forces individual story arcs to unfold at high-speeds with the most predictable fashion possible. If an episode starts by looking at something from the perspective of one of the characters that they haven't focused on, get ready, because here comes that person's entire backstory. The overarching issue is a question that all writers need to ask themselves when they create any kind of conflict, and that question is, "why now?" We, the audience, need to know why something is happening at that exact moment, or it feels overly orchestrated and highly unrealistic. Which is why, when Yuri has been searching for something lost in space for X amount of years, and then he happens to find it in the exact episode we learn about it, it ends up feeling like a fresh pile of bullshit.
Fortunately, in the latter half of the series, Planetes takes a much more linear story-telling method that causes the later episodes to be written significantly better overall, however, it is still far from being without its faults. One of the "nice, little bows" that Planetes tries to wrap itself up in, is an ending that pitches the idea that everything in the universe is connected, and while I think that conclusion was little too forced down our throats, it has the much bigger issue that there are simply too many coincidences in this anime--like characters repeatedly just happening to bump into each other (ya know, because coincidences are "better" than giving a logical reason for two characters to be in the same place at the same time).
Speaking of characters there are a number of issues here too. Obviously, the aforementioned issue of the episodic storytelling in the beginning does negatively impact the development of the characters, and forces their growth to feel forced and fake. Additionally, the main duo, Hachimaki and Tanabe have their ups and downs as individuals, but they also feel kinda forced as a relationship. The series starts through the eyes of Tanabe, an obnoxious girl who believes in over-idealized realities and is-in my opinion-the worst character in the show. However, I will say that she was certainly more annoying in the earlier half, but the opposite could be said for Hachimaki. Hachimaki felt like the better-written and more enjoyable character of the two, but he ends up treating Tanabe bad enough at the end that I feel like his actions should've rightfully torn the two apart permanently, but of course, a coincidental meeting happens, where he swerves of the road, and then who does it happen to be that is nearby at the time, of course it's Tanabe! Regardless of that, I'd say both of them are still written in better than Yuri or Edel, whose characters are basically forced down our throats. Lastly, I think my favorite characters have to be Robbie and Fee; I say, although I ironically don't like Robbie that much, HOWEVER he does have some of the most genuine-feeling growth, and watching him transition from a corporate drone to a man, who takes pride in what he does because he wants to be the kind of man his children can look up to, is quite enjoyable. Fee does not have the same levels of character growth Robbie has, or even close. No, actually I'd say Fee has one of the least built in stories (next to the chief, who is the only member of the debris section who doesn't have any kind of character growth), but regardless, Fee is a smoker ... in space, which as you can imagine, is quite the hard habit to maintain in an environment where air is incredibly valuable. While this isn't all that interesting by itself, it is quite entertaining to see just how far she'll go to have smoke (which in my opinion, is far enough to create the best episode in this series).
TL;DR -
Story: 6, Has some major issues in the structure in which its told; the overly episodic approach for the first half of the series was basically Planetes's attempt at shooting itself in the foot, and lets just say it definitely shot off a toe or two.
Art: 7, Standard quality and consistency.
Sound: 8, OP and ED suited the anime, but weren't to my personal tastes. OSTs were solid, and the VAs (English DUB) did a pretty good job.
Character: 7, None of the characters are fundamentally poorly written, however there are several issues here.
Enjoyment: 7, If I hadn't been so focused on what could've made this better, I think I would've liked it more.
Overall: 7, Definitely enjoyable, but shoots itself in the foot, and prevents it from being something truly great. Would still highly recommend.
