My point is that I try to make sure there ARE reasons. I get inspired by reading random things, and in the case of Iris, it happened to be a trope description for the Staff Chick; I started pondering why so many characters of that overall type happened to have such similar personalities. I then started thinking about the sort of person I've never seen a Staff Chick be, and one of the things that crossed my mind is that even in settings where everyone is messed up in some huge way, the Staff Chick's neurosis is usually shyness and anxiety. So I thought, "What could you do to give her a different mental health problem?" That's the point where the weird little corner of my brain that adds trope hyperlinks to everything brought up that whole archetype of the girl who looks cute, but if you catch her at the right/wrong time turns out to be frighteningly unstable. Then some other corner of my brain went, "Hey, that actually sounds sorta cool!"

My point is that while she was inspired by the two tropes, by now Iris has a lot more to her than that. I could describe her as a Yangire White Magician Girl, or I could describe her as a young woman with a natural talent for healing magic and a family history of schizophrenia who saw both of her parents killed by undead with her biomancy-vision on, and the massive emotional and psychic trauma from that incident triggered her into full-blown disorganized schizophrenia. She's in a setting with little understanding of psychology as a discipline and certainly no antipsychotics, so even though years have passed since The Event, she still regularly slips into word-salad babbling and often says things that other people find disturbing without realizing there's anything odd about it. She clings to her magic almost as a security blanket, playing with raw biomantic force constantly because she likes to watch the little blue lines move (and their absence reminds her of the forces of anti-life animating the monsters that killed her family). She loves dogs, has been known to try to make "improvements" to local wildlife (She once found a sparrow with a broken wing. It now has an unbroken wing, three eyes, and rainbow pinfeathers.), and has either panic attacks or violent-screaming-terrified-rage attacks when faced with undead. And she's a Cute Psycho White Magician Girl.

The closest equivalent to how I think about tropes is... well, has anyone seen the Dresden Files RPG, and how it describes characters via "Aspects"? For example, Harry has the Aspect "Wizard Private Eye", but no one's saying that's all he is. And hell, half the Aspects in the Who's Who listing pretty much are tropes; Morgan's "Zealotry in the Pursuit of Justice is No Crime" means the same thing as the trope "Inspector Javert," and Thomas's "I Must Fight My Demon" could be easily replaced with "Enemy Within," for example.

Speaking of Jim Butcher, he's said that he once got in an argument with a friend about whether you needed a good idea to have a good story, or whether you could write a good story based on a bad idea. His friend dared him to write a good story based on a stupid idea of his (the friend's) choice. Butcher took him up on it. The idea: the lost Roman legion, with Pokemon. The result: the Codex Alera, one of the most crazy-awesome high fantasy series ever.

tl;dr: I don't think the source of an idea automatically detracts from it, whether TVTropes or anything else. Or, to quote the SCP Foundation:
*The Vampire Diaries Insight: "There's no such thing as a bad idea. Only poorly executed awesome ones."
**Clef's Addendum: "However, some ideas need to be executed perfectly to cross the line into awesome."

The author of that fic is a known troll. He likes to torture us, it would seem.

It's far more than just the tactics, there are WW1 references all through the Guard;

The tank designs themselves are more reminiscent of WW1 era vehicles than modern machines, with the tracks that circumnavigate the entire chassis, and particularly the side mounted weapon sponsons available for the Leman Russ (and various other Forge World tank models).

They're one of the few armies those infantry regularly have fixed bayonets (the others I can think of being possibly Orks, and Chaos, although they usually have a 'spiky and bladed' theme in total, so I'm not sure it really counts).

They have elite infantry units called Stormtroopers.

Several of the worlds mentioned in the Codex and novels have conscription.

There might be some more, but I can't think of it right now.

Due to the parallels between IG and WW1 armies I have actually considered trying to build a trench system for my gaming table, although haven't taken it anywhere yet as I haven't worked out how to do it fully.

Run out of reserves? Pfft! SEND IN THE NEXT WAVE! (No, really, there's an IG commander with a special rule that allows him to remove damaged Conscript squads from the board and redeploy them, at full strength, from the board's edge. At no extra cost.)

Also, Creed and his Infiltrating Baneblades/Warlord Titans.