I agree with a lot of what has been said. It's not like I go back to watch seasons regularly, but the impact and impression I have of each is generally reinforced after catching some reruns.
S1 is what got me hooked into the series. But what I loved about it was not necessarily that it was dark, but that it was
cohesive, and knew who and what it was.
It felt like a world, it was a believeable version of New York, it was grittier, it was often literally and metaphorically dark, but it's not like it was devoid of humour in the midst of all that. I think there was a learning curve within the season that I wished had been permitted to continue the following year instead of having what felt like a drastic and involuntary make-over imposed.
The best of the darkness was lost altogether to brightening, smoothing and polishing everything up. I think that's where a lot of interest was lost.
I think the show's lost it's originating identity and overall compass and has been flitting about tinkering with so many things to retain a certain cachet, top down instead of from the street up. It is rather anonymous in it's context now. It doesn't seem to matter where it is, as it doesn't seem to flavor the show anymore. Not enough. New York is a pretense, no longer a presence.
S1 seemed more secure in it's vision, of it's world and it's characters, and felt like it just went about doing it's own thing. That is something else I miss. There have been eps that I really quite liked since, but taken as a whole, I think S1 still does outstrip the rest, and that's really due the writing and cohesiveness than how dark or blue in hue it was. If I simply consider S1 a separate series and the following as spin-offs, that kinda helps...
I think the writers and execs need to go on a retreat to the boroughs and renew their inspiration and vision of the show.