I didn't actually follow any of this advice, but this is what I've seen that I know absolutely would not work for my family.
For me, it was buying refillable mugs for a family of 5. We never used them enough to make up for their cost. The drink stations were too far from the room and no one wanted to carry them around the park all day just so we could refill them on the way from the bus stop to the room in the evening.
We never bought the mugs until our BC trip. On the other trips, the station was so far away from our room that we never would have used them. At the BC, the station for refills was just a minute's walk from our room, and we got a lot of use out of them. I would never carry one into the parks, but we used them in the morning, during afternoon breaks, and in the evening.
don't upgrade to deluxe dining plan, take the basic plan for free. for us the upgrade was totally worth the money. we paid for no snacks or meals out of pocket. yes at times it felt like a lot of food, but for two adults going all day in the park two meals and one snack just does not cut it. also it goes by nights not days, so we do a full day on arrival day and stay until at least 3pm departure day. when you do the math its 10 meals for almost 6 full days, we would have spent more out of pocket to supplement the plan once we arrived.
Our experience was the opposite. The basic plan was more than enough (in fact, we always end up using unused snack and CS credits to take food home).
packing so much stuff you have to pay fees to fly it down.....bad advice!
bringing backpacks and big bags of junk into parks daily-ugh!
I bring the tiniest bag I can manage that fits my stuff- and buy lunch while I'm there...I don't lug bags!
Bringing bottled water into parks- the true moneysaver is getting free ice water anywhere in the parks- and NOT lugging extra stuff around! (see above

)
I guess I have a theme.....minimalism!
Oh, yeah! I'm not going to be a pack mule on my vacation. Flying cases of water to your resort? Carrying lunch, drinks, and a change of clothes? No way. I know this is a great idea for some people, but I'd rather stay home!
a 3rd pair of shoes solely for use in the room;
I've never seen this one before. What's the purpose of a pair of room-only shoes?
I agree with this. I cannot understand why people want to bring food to eat breakfast in their room. There's hardly any space, unless you're off-site or in suites with a kitchen. I don't want crumbs and food everywhere. And having to clean up stuff afterwards? Waste of time to me.
I bring boxes of Poptarts or granola bars, and we get some pastries in France. It doesn't take up excessive room, it's not messy, and there's nothing to clean other than throwing away empty packages. And it saves so much valuable morning time!
And if I had a kitchen? I sure as heck don't want to COOK when I'm finally on vacation!
I totally agree with you on that one. Even the above-mentioned jar of pasta sauce and frozen meatballs is more than I want to deal with on my WDW vacation. Not to mention doing the dishes. I love having a kitchen for a leisurely beach vacation, but my time is more precious at WDW.
As for a TS meal wasting 2 hours or more of park time? Well, that's personal preference. To us, eating a TS meal every day at a beautiful themed restaurant was part of our vacation & Disney experience!
This!
To me, it is far easier to toss in a load before getting into the pool, then it is to wash load after load after load after load when I return from my vacation. Literally, it could be an all day affair!
Seriously, I never spent more than 10 minutes "doing" laundry. What does it take? You toss it in (2 minutes). You move it to the dryer (1 minute). You remove it from the dryer and fold (maybe 5 minutes?). Big deal.
My laundry takes a lot longer. I hang my and DD's cotton shirts to dry to prevent wrinkling, so I'd have to take the time to do that, and I'd have to have hangers. Also, I prefer to stay onsite, and being able to do my laundry isn't worth being offsite.
For us, what did not work was pre-buying souvenirs and taking it all with us on the trip to prevent the gimme's. 1 my boys never had the gimme's so I was lugging around stuff they did not want anyway. 2, it was all dollar tree stuff and not the best quality, I remember waiting for the parade and proudly pulling out the glow sticks and 90% of them not working! 3, If my Ds's saw something they really wanted, they really wanted it and no substitute out of the bottom of the stroller was going to do.
This is always the tip that mystifies me the most. If my child sees something she loves in the park, she loves THAT. Me drawing a cheap toy from home out of my bag and saying "Surprise! Here's your souvenir for the day!" is not a substitute. It's not a souvenir from her trip, it's a gift someone else picked out and happened to give her at WDW. We have a souvenir budget and save our shopping for one day, and that works for us.