What is it?

My broadest interpretation is that gender nonconformity as a whole is anything that subverts widely held expectations, unconscious or not, that are based on beliefs about biological sex that have nothing to do with physiological differences between women and men. I see gender conformity as an adherence to such expectations, however that may manifest.

What does it look like?

Gender nonconformity can look like a lot of different things, whether through fashion, beauty, language, mannerisms, hobbies. When speaking of someone being GNC, it seems like it's a broad combination of characteristics for many GNC people. The subversion appears to often come across as something unusual or irregular via feelings of surprise, shock, confusion, disgust, curiosity and other feelings that at least momentarily stop the self-talk that would otherwise go unnoticed. Gender conformity is something that most people don't seem to notice or consciously pay attention to; it appears so mundane that instead we focus on the details about something or someone without (usually) consciously thinking about or attributing it to biological sex or sex role stereotypes.

What does it feel like to you?

It feels like nothing--it feels natural. But I used to feel bad about it because it was the reason I was harassed and bullied so much before transitioning. Just the thought of having to be seen by most in society as an exceptionally feminine man again makes me feel anxious and nervous. So, it feels normal, but as a by-product of not meeting society's expectations about males that leads to negative consequences and outcomes, it makes me feel bad about myself.

For me personally, it is a bit complex since I don't believe I'm really viewed as gender nonconforming so much anymore since most people who don't know my history seem to believe I'm female. So I suppose my female-attributed characteristics, traits or mannerisms don't draw attention to myself. One of the biggest reasons I transitioned was so I could fit in better, so what would be considered GNC expression otherwise is just unremarkable.

Is it a deliberate act of adopting gender norms of the other sex or is it normal human preferences being expressed without the tightlacing gender puts out

An expression of gender nonconformity could be deliberate, but I think at its heart it really is about natural human preferences.

Is aggressively adopting the norms of the opposite sex actually nonconforming?

I suppose it might depend on who you ask and what the intent is in doing such things. Adopting norms of the opposite sex would be nonconforming from the perspective of the 'original' sex, but then it would be gender conforming from the perspective of the opposite sex, maybe. I guess when you say 'aggressively adopting' my first feelings are that it isn't naturally gender nonconforming in the slightest.