I have two main issues with slice-of-life shows: Most have extremely boring/over-used settings (some sort of school or a house/neighborhood, but always in upper middle-class, modern-day Japan), and have a cast almost exclusively made up of cute teenaged girls. Because of this, I find most slice-of-lifes stagnant and/or derivative.
So-Ra-No-Wo-To fixes the first issue. The setting was by far my favorite aspect of the series. It was imaginative, richly-detailed, well thought-out, and most importantly wasn't set in the present or in Japan. The in-show culture was just a giant hodge-podge of existing cultures, but it's interesting, and makes sense if you think about how the world could have gotten the way it did. Additionally, I can't think of another anime that applies slice-of-life to a military setting, I was very impressed by how well this turned out.
Unfortunately, this makes So-Ra-No-Wo-To's failure to remedy the second issue that much more disappointing. This is a military setting. That's pretty gender-neutral. If anything, it would allow for more male characters than female (every other squadron shown was certainly much more male-heavy). So why, WHY are all the main characters cute girls between the ages of 14 and 18?! It's completely unnecessary, and, I would argue, hurts the overall quality of the series (at the very least, in terms of believability- Noël's backstory could have been salvaged for me if she hadn't been a small child in her flashbacks, that totally killed all realism). I mean, the characters themselves were great (for the most part). At the start, it seemed like it was just going to be a cast of clichéd stock characters, but the ensuing character development was very well done and rounded everyone out nicely (Kanata didn't turn out to be a clumsy idiot, Noël shows great displays of emotion, Filicia ended up being really edgy, etc.). Granted, sometimes these characterizations are taken too far (mainly with Noël and Felicia in the last couple of episodes), and Rio angsted a bit much, but overall I found them refreshing from most teenaged girl slice-of-life protagonists. So why must they be teenaged girls? I can only assume it's to pander to the rampant moe fans in the audience, as there are quite a few instances of completely unnecessary moe-ness (why the dickens does Noël's pyjamas include zettai ryouiki? And I assume this was the reason for episode 8. Perhaps some people find such things cute, I, for one, found it rather unsettling).
In the second half of the series the drama kicks in, which was sort of a mixed bag. The majority of it was pulled off well, except for the slapped-on deus-ex-machina finale that reeked of unrealistic cheese (blech). Your best bet for enjoying the final episode is to take everything at face value. Because if you actually think about some of the things that happen, it's immediately evident that they're silly and/or downright impossible.
Comedy is usually the saving grace of slice-of-life anime, but So-Ra-No-Wo-To was sorely lacking in this department, even before the hard-hitting dramatic parts come up. Not really a major deal-breaker, because it had other things to offer, but if you're expecting humor, you may want to reconsider, because I can only recall a few short instances where I found anything to be funny in this show.