OK - I'll weigh in.
Now I love me a dirty plane or a corroded tank or even a muddy Subaru, BUT, only where it's appropriate.
I'd not begrudge someone from making the model how they see or want it to be but that doesn't stop me from disliking it just as I don't expect all to fawn over my outpourings.
One thing I find annoying is what I call "The Verlinden effect" where every crease, fold, panel line or bump is outlined in a dark colour beyond reality.
Yes there are examples of heavily weathered objects, but in general, for example, you can't see the panel lines at 40 paces so if the scale of the model is say, smaller than 1/72 - ease up on the friggin black lines !
Airliners with black lines all over them just look horrid IMO and totally unrealistic, but if it's appropriate ( according to that photo you took on your last Thailand junket) then fair'nuff. Still looks wrong but.
Conversely - if I see a Tornado with a clean tail I'm immediately suspicious - have you seen how filthy they get after just one flight ?
Then there's the tank brigade and their Shermans and Tigers that look all rusted and forlorn. Most of these vehicles were new at the time of use so unless it's a T34 found languishing in the back row at a Syrian warehouse thirty years down the track it's not going to be rusted to the hilt.
Oh and another thing, M113s are predominantly made out of aluminium with only certain parts that rust. Metallurgy is a thing.
Did you know that not all exhaust stains are black ?
That's about as dirty as I like my wartime aircraft. ( Yes the aerials are wrong - there's a reason for that which you can read about elsewhere )
Tell a lie - this Beaufort I really made dirty - but that was based on some research on what was happening in PNG at the time.
Conversely I made my 1/32 MB339 as dirty as I observed a modern RNZAF aircraft to be whilst being operated, potentially to full capacity every second weekend.
ie - not very.
So my thoughts on the matter.
Less is more.
Black is never black.
If you can model NM finishes effectively you are a better man than I.
Pastels are good.
Mud effects the wheel wells as well as the wheels.
Learn about nuln oil if you're making figures. ( It's like Verlinden-lite)
Preshading is bollocks.
OK preshading sometimes isn't bollocks but if you're laying on the panel lines with your eBay purchased Gundam marker and filling in the panel line via your art shop Kuretake pen with your Winsor & Newton black ink and dry-brushing from a wet palette using Citadel chunky paint then why the hell did you preshade ?
I got you good by saying preshading is bollocks yeah ? I don't really think it's bollocks.
Sporadic misting via an airbrush using Testors dullcote with just a hint of a darker shade or lighter shade of the subject colour is a good way to subtley "weather" a panel.
Don't get me started on gloss finishes - mainly because I'm not an expert. I'd just say "go Pledge it fanbois" and wait for the fallout.