Oh, goodness — everything. Film, camera, lenses, lighting, style, fashions and trends, whether or not photography is art, everything we see today. But the "forums" were... more literal forums — people getting together in person to discuss in photography and camera clubs. And, instead of the blogosphere of today, so many periodicals. To get a taste, do a Google Books search for "photography" in the 19th century. This will return some scanned and digitized annual collections of these journals. Of course, you can also choose later dates, but I think it's interesting to go back... basically to the beginning. Browse through though those, and you will see that the more things change, the more they stay the same!

For a quick sample, here's the index from The American Annual of Photography, 1887:

• Abnormal Chloride of Silver.
• A Cry for Simplicity.
• Alum in the Hypo Bath.
• "Amateurs," "Professionals," or "Photographers."
• Argentometer for Controlling Silver Baths.
• A Talk to Beginners.
• The Origin of Blisters in Albumen Prints
• About Bromide Paper
• A Home-made Tourist Camera and a New Tripod-head
• Carbonate of Soda Development for Platinotypes.
• Collodion Emulsion.
• Colored Screens in Out-door Photography.
• Composition in Landscape Photography.
• A Convenient Carrying-box.
• A Copying and Enlarging Closet.
• Dark-rooms.
• Dark-rooms for Photographic Clubs.
• The Detective Camera for Newspaper Photography.
• Detective Cameras.
• The "Detective" for Artists.
• A Convenient Developer for Travelers.
• A New Developer
• A New Developer (Second article)
• Does it Pay?
• Durable Sensitized Albumen Paper.
• Drying Negatives Quickly.
• Enlarging and Reducing with Ordinary Camera.
• The "Equivalent Focus" Not a Constant.
• Figure, Landscape, and Combination Printing.
• Focal Adjustment in Detective Cameras.
• Focusing.
• Frills and Blisters.
• Genre Photography.
• Heliogravure and Galvanography.
• High or Low Art.
• Hypo in the Developer.
• A Method of Ink Transfer.
• Iodide of Silver in Gelatino-bromide of Silver Emulsion.
• "It's no Jist Ready Yet."
• Japanese Paper for Photographers.
• Lantern Slides and Transparencies.
• Large Telescopes for Celestial Photography.
• Magnesium Light Experiments.
• Marine Shots.
• Memoranda from My Reading and Practice.
• Mixing an Emulsion with Ammonia.
• Mounting Silver Prints in Optical Contact with Glass, and Ornamental Borders.
• Musings by the Way.
• My Dark-room Window.
• "My" Developer.
• Objectives for Photo-micrography.
• Observations.
• Odds and Ends.
• Old Friend, Keep Young.
• On the Character of Optical Glass.
• On the So-called Optical Sensitizers.
• Orthochromatic Photography of Paintings.
• Orthochromatic Plates, Experiments with.
• Over-Exposure and the Means to Correct It.
• Photo-copper-plate Printing and Steel-facing of Etched Copper-plates.
• Photographic Chips.
• Photographing in Japan.
• Photographing in the Arctic.
• Photographing Machinery.
• Photography by Lamplight.
• Photography in the Navy.
• On the Photography of Solar Eclipses.
• A Simple Arrangement for the Production of Photo-micrographs.
• Portraits in Imitation of Old Paintings.
• Practical Hints on Obernetter's Method of Copying Negatives.
• Preparation of Photographic Gun-cotton, On the.
• Printing, Suggestions on.
• Reproducing Architectural Drawings.
• Results.
• Retouching Gelatine Negatives with Red Chalk and Black Crayon.
• [Editor's] Review of the Year.
• Reversing the Print.
• Silver Printing.—An Amateur's Experience.
• An Amateur's Routine of Silver Printing.
• A Simple Time and Instantaneous Shutter.
• Some Thoughts of an Amateur.
• A Home-made Stereopticon Outfit
• Stereoscooic Pictures.
• Tannin Plates for Lantern Slides.
• Temporary Binding for Photographic Journals.
• Theatre Photography by Electric Light.
• The Elimination of Heat from Condensed Light.
• The Lightning Photographer: A Sketch of the Future.
• The Undeveloped Side of Photography.
• Why Study Art?

I do kind of have to question your premise, though. I don't think sensors are an overriding obsession — on this site, we get at least as much worry about lenses and other gear too. Just like the above. There really wasn't one main thing then, just as there isn't now. That's actually part of the fun and appeal of the hobby — there are obsessions available for everyone, from deeply technical to highly philosophical.