I believe James Cameron's Aliens, released in 1986 was the turning point. From Metropolis, Things to Come and various George Pal movies to the original Outer Limits and Twilight Zone, science fiction films and television most of the time had a sleek, sophisticated aesthetic. Even though Ridley Scott's original Alien ('79) and later his Bladerunner ('82) had gritty visual elements, he is such a superior visual stylist that even these worn, industrial and sometimes disgusting environments still looked great and seemed futuristic and compelling (Paul Verhoeven's Robocop fits this pattern also). With Cameron's Aliens we got the dingy grey, militarist action-adventure overlay on the genre and it certainly has stuck. His original Terminator along with the cult-hit The Hidden both had real-world grit but were very stylistic and thus felt more old-school. But action-adventure now trumps old-school sci-fi most of the time. Partly this is due to the overall dumbing-down of culture - scientific concepts fly right over the heads of the average fan - and probably the executives greenlighting these projects too.
I think sci-fi fans today fall into two major categories - those who prefer the futurism and sleek aesthetics along with a focus on technology and profound ideas, and the new majority which likes action-adventure dressed up as science fiction. One of the best of the latter category is Predator and one of the worst is Event Horizon (which drifted over into another sub-category: sci-fi/horror).
Of course there is nothing wrong with creative mixing of genres, fans merely ask that it be done well. Give us quality and we will read, listen and watch (and buy your products).
Which brings me to a dichotomy I see in Battlestar Galactica. It is this sleek, beautiful and sophisticated visual and intellectual entertainment experience - made more whole with its ancient past and conflicting religions subplots. Yet it is also a gritty, dank war story replete with war story cliches. The two are not mutually exclusive, of course, but the biggest creative challenge for the show and for the viewer is processing these often out-of-sync story elements. For example, many fans count as a favorite the boxing episode Unfinished Business - which I loathed. Some of the backstory elements of the episode were good, but overall it was a useless episode with ridiculously excessive machismo that really served no function in the exposition. An episode which showed how conflicts and resentments built up over the course of the fleet's confinement and exodus would have been more interesting.
As the final 10-12 hours of Battlestar Galactica wraps up, these two competing story elements (I don't personally feel that they compliment very often) will have to weave together for the finale. There seems much more war story militarism to come (a fractured fleet, the Zarek coup, the attack from Cavil's forces) and gritty, bitter greyness thematically ... but I am hopeful the grand ideas of the show will be given their fair share of the focus. According to actress Grace Park (Athena/Sharons/Boomer) there is one episode in which it is all explained - I'm predicting the mad-scientist-explains-it-all-to-you-scene (or toothless homeless man such as in Twelve Monkeys). Hopefully the post-series movie The Plan will have more of the sleek aesthetic elements since it deals with the Cylon perspective, but they assigned one of the weaker writers to that project so maybe not.