Mechanics should be off-topic
I see a lot of mechanical questions, as in, "How do I fix my mower?" I think this type of question would be better served on https://mechanics.stackexchange.com/?as=1 and I say this as a small engine mechanic. Just because a piece of machinery is used for a landscaping purpose, doesn't mean gardeners and landscapers should be qualified to repair it. Should a cook be qualified to repair a stove? Shall I ask on the Seasoned Advice site how to repair my blender? This is what the maintenance department would be for. Furthermore, if lawnmower repair questions are on-topic here, then I should be able to ask how to repair my go-kart, log-splitter, generator, snow blower, water pump because the engines are the same Briggs n Strattons, Kholers, Hondas, etc, etc.
On-topic lawn mower questions could be, "How high do I cut my grass?", "When should I cut my grass?", "How do I cut patterns into grass?", "What's the best mower to mow this type of plant?", "How can I make my mowing experience more comfortable?" Things that have to do with the plant or the operation of the machine are things gardeners would be qualified to answer. Even "How do I sharpen my mower blades?" might be ok. Technically, its a repair question, but so many landscapers sharpen their own blades that I would expect a landscaper to be able to answer the question. "What kind of mower blade should I choose?" would be on-topic.
"How do I change my blades?" Consult your manual.
"How do I change my mower belt?" Consult your manual.
"How do I change my oil?" Consult your manual. "A spring fell off, where does it go? Consult the exploded diagram in your manual.
Chemistry not related to a plants should be off-topic
Other off-topic questions would be fuel related as gardeners wouldn't be expected to have extensive knowledge of chemistry. This would be more of a mechanical question (though could be considered off-topic there as well, in which case chemistry would be my next guess) because mechanics often study fuels in order to increase performance or efficiency. Gardeners typically do not. Many gardeners use no fuel whatsoever. However, show me an engine mechanic who never uses fuel.
Most farming should be on-topic
I don't see anything wrong with farming questions. If I can give a recommendation on 1 plant, I can give one on 1000 plants of the same variety. We've already eliminated biology, cooking, chemistry, diy, and mechanics... if we eliminate farming, what will be left to ask? Questions like "What combine should I get for my 1000 acre wheat field?", I don't think any gardener could answer that. Anyway, it seems like a question that would never be asked or ever be answered if it were to be asked. Also, "What should I plant this year to make the most profit?" or "How much fertilizer should I apply to maximize profits?" Those seem like math or economics questions to me.
Pavers, blocks, retaining walls, gnomes
Regarding pavers, blocks, retaining walls, gnomes... I think the gardener should be qualified to answer questions on how these things should be structured or placed for the benefit of plants. The actual construction method is a diy question. "How tall do I need to build my retaining wall?" is ok, but, "How do I build a retaining wall?" is not.
"What can I add to my soil to make my food more nutritious?" I think a gardener should have learned the answer to that during his studies of soil and plant nutrition. "What's the most nutritious food?" is off-topic. A possible exception would be if discussing the decline in food nutrition over time due to bad farming practices or hybridization. A gardener may know a bit about this (personally, I think he should).