Let me start by throwing these four examples out there...
What you likely noticed first is an increasing level of quality as time goes on. Comic Fan is mimeographed on cheap paper using decidedly amatuer artwork, while Hero Illustrated has four-color offset printing with art by one of the most professional comic artists of all time. The single staple holding Comic Fan together is replaced by two spine staples for Comic World and Comic Reader and eventually a perfect binding comfortably snug in a sealed polybag for Hero Illustrated. A lot of that, not surprisingly, has to do with the technology that was available, but more to the point, it was the cost of the technology. Offset printing was around in the 1960s, but it wasn't commercially viable for the type of work being done on fanzines. And while it was still probably too expensive for a single individual in the 1990s, it had lowered enough that a small group could pool their resources and put out a very professional looking magazine.
That part seems pretty self-evident, I hope.
But there's another aspect you can't really see just from the covers. Comic Fan was almost completely original comic and prose stories. Comic World was more of a retrospectives zine focusing on beloved (but by then mostly forgotten) characters, but it also rans some small classifieds. Comic Reader was more of a news and interviews thing with both classifieds and full-on ads. I believe they also had a few comics pieces as well. Hero Illustrated was a combination of news, interviews, speculations, commentaries with a good number of ads, as well as promotional items that might also be included in the polybag.
What you see if you were to flip through these different pamphlets is a change in perspective. While the earliest zines focused on sharing the publisher's interest and/or utilizing the format as a creative outlet, the notion of subsidizing the costs took greater and greater focus. A few classifieds at first, then regular advertisements, and then promotions that are effectively out-of-scope for the production of the magazine itself and require a solution like polybagging to be included. The very reason for the magazine's existence shifts from a simple love of the medium to an ongoing financial concern.
You can see this played out in other venues as well. Cosplay, conventions, filking... they all started exclusively from a love of comics and are all now used as money-makers. People now have "earning a living from cosplay" as a career goal.
Comics isn't the only environment this plays out in, of course. The same thing has happened with science fiction, fantasy, rock and roll... heck, sports is the very definition of turning a fun pastime into a hugely profitable business.
But I think that it's not always evident to the fans themselves. They complain about how San Diego Comic-Con is so commercialized now, or how they miss the letters pages of old. These things don't suddenly change overnight; they all evolve gradually over the course of years. It's only by paying attention to the small aspects of commercialization that you see the slow shift from a "more pure" love of the medium to an effort to earn a living that extends from that love.
I'm not passing judgement here; you can plainly see I've got a few ads running down the side of my site after all! But I think it's worth pointing out so that you can be more aware of that shift, and decide for yourself whether the love that was there at the outset is slipping away or not.