Wells is fine and good, but he's ancient history. And science fiction went through a couple of metamorphoses in the pulp years of the 1920's and 1930's. It was during this period that space opera started (mostly through the works of Burroughs and Smith).
The early dominant personality was Gernsback, who was willing to entertain monster and fantasy (of a sort) as well as "science" sf. The start of Amazing Stories with Gernsback as editor in 1926 is considered by most to be the "birth" of modern science fiction.
When John W. Campbell, Jr. became editor of Astounding Stories (which he renamed Astounding Science Fiction) in 1937, the "science" type of sf story came to the forefront. His power in the sf world was immense as late as the early '60s and continued to be significant until his death in '71. Campbell was the editor that found and gave us people like Asimov, van Vogt, Heinlein, de Camp, E.E "Doc" Smith, and others.
Campbell had very narrow tastes and attitudes. Thus, the "golden age" sf has some very characteristic traits, the most prevalent being the artificial and strained relationship between the sexes. Frankly, on that topic Campbell was very much a prude. By the early sixties, that was starting to be a serious impediment to some of the stories that people wanted to write. In particular, Heinlein's Stranger in a Strange Land could not find a publisehr for quite a while. Campbell would not serialize it unless Heinlein took out the "sex" scenes (laughable, given the they would hardly qualify as such even ten years later). Heinlein, quite rightly IMHO, said they were essential to the story and would not remove them. But when it was finally published, it started a floodgate of "new" sf.
In the '60s, following the lead of Heinlein, topics other than science started becoming more prevalent in sf. This period gives us works like A Clockwork Orange, Cat's Cradle, Bable 17. While "sf" could still be read as "science" fiction, more and more of what was being published under that ruberic had little to no science in it at all. Towards the end of the decade, Harlen Ellison edited Dangerous Visions and with that, science fiction ceased to be a genre and became a subset of speculative fiction (although many still use the old terminology).
It is not the change in the subject matter that has turned me away from sf, or most fiction for that matter. It is that the writing has gotten poorer, possibly because the intended audience has become more ignorant (or, perhaps I should say has not had its ignorance lifted because of the failure of the educational system). One needs only follow the Pern series, from its birth in Again Dangerous Visions through nine plus books devolving in cute Dragon Lizzards and the quaint behavior of singers.
Given that the general audience has not the sophistication of a well educated twelve year old of half a century ago, it is no wonder that the pap that is now published is not found nourishing for the developed adult mind. And that is true in many fields other than sf. The very complexity of a Le Carre novel which made them best sellers fourty years ago make them all but unsellable now.