Strike Witches is a very polarizing show. Essentially, it is WWII, except all the fighter planes are young girls carrying guns, and instead of fighting each other they're fighting aliens. It gets a lot of flack for fanservice, a fair bit of which is justified, but the underlying war story's pretty well done.
To be more explicit it "gets a lot of flack" because it massively panders to paedophiles.
That would be why the most often depicted examples are massive breasts. No, wait, that doesn't make any sense at all.
That was probably more in reference to the fact that a large portion of the cast are underaged girls who spend most of their screentime running around without pants for literally no reason at all.
While not explicitly stated (at least in the anime, I haven't examined any of the supplemental material either), it seemed fairly obvious to me that the lack of pants was so they could wear their airplane parts. Of course, that doesn't explain why pretty much no women in the universe seem to wear pants, unless there's some sort of war-hero based women's lib movement going on.
That said, I'm pretty indifferent to fanservice unless it's extremely gratuitous (High-School of the Dead is one of the only shows I can think of that really annoyed me, mostly the guntits scene), and absolutely despise it when things that should be seen aren't (unless it's a comedy show poking fun at that kind of thing). Things like the Magic Skirt, pixellation/black bars, or absurd amounts of steam really piss me off because they jar my suspension of disbelief so much. It isn't about missing out on seeing something, it just seems so fundamentally wrong to, for example, have a girl hanging from a rope by her feet with her skirt firmly in place instead of obeying gravity, and throwing something on the screen to block something out is basically beating you with the fourth wall.