Part1 I know some people don't understand or disagree with me on this, but I've always felt with certainity that Oliver's lack of compliments towards Felicity's apparence has always been deliberate and fitting. Whether it's Stephen's or the writers doing, I think Oliver may feel like these passing compliments or comments are nearly not enough, or he just can't let them out.
Part2 Nevertheless, he is incapable of hiding his appreciation and devotion from his eyes, which IMO somehow makes the reaction to Felicity’s outfits and looks much more. Doesn’t mean I’m not all for him actually voicing his admiration in s4 though.
A very interesting point, anon.
(And of course I have to ramble about it, I apologize for this randomness. Think of this as a look at what my brain is like when I’m thinking on a story: a hot mess.)
Part of the magic of Olicity was how much was said without actually vocalizing anything. They have entire conversations with their eyes alone:
(Yes, this is me using Olicity gifs that have next to nothing to do with the question. Are you arguing with me about this?)
It’s 100% more intimate and fascinating than if he did say something.
Now, why doesn’t he say anything?
Well, Oliver isn’t much for vocal appreciation when it comes to appearances, in my opinion. In fact, I would argue that outward appearances are pretty damn low on his list of things to care about. That doesn’t mean he doesn’t care about them, because he’s a red-blooded fella, of course he notices things, but it’s not high on his priority list. He’s learned the hard way that people aren’t what they seem on the outside.
(Also, the guy is stoic as fuck and incredibly tactile, that is predominately how he shows his appreciation - that lucky goddamn Felicity.)
I think vocalizing things when it’s not entirely necessary makes him uncomfortable. Pre-island Oliver was incredibly vocal about appearances, because it got him places and it got him things; he was also very aware of how pretty he was, to the point of being cocky. Post-island Oliver plays everything much closer to the belt - he’s very, very aware of the power of words and perception, and he’s very aware of the power of his looks and, at the same time, lack of power, depending on the situation. He’s not that guy anymore, so it’s not something he partakes in unless it’s to his advantage in some way.
I think that is why he’s so quiet about his appreciation more than anything, especially when it comes to Felicity because he’s always been incredibly upfront and honest with her. He’s not that guy anymore and he didn’t have to put on the show around her, so it wasn’t something he did (at least outwardly).
But… let’s take a gander down this road anyway, because I’ve thought about it for fanfiction reasons. Here’s my quick perception of Oliver Queen in this regard, that applies to both canon and building his character in fanfiction:
Why doesn’t he say anything?
Because the dude’s head over heels in love with this woman.