If we are talking kids (who unlike today, were the majority of users of the trains) I think the biggest complaints would be they would like to be able to afford more track and switches, have more engines and the neat operating accessories, rather than waiting to get them as presents at birthday and Christmas, and of course have room and resources to build a big layout (in a sense, this is kids realizing the trains are expensive).
Adults in the 1950's:
-Kids are ungrateful, you work hard to give them things, and they want more
-Kids aren't like we were, all they want to do is sit and watch TV or play. Why, I walked two miles uphill, both ways, in knee high snow to go to school and I worked three jobs, too (obviously, the parents of the 1950's grew up during the depression, so some of this was not hyperbole).
-Kids are listening to that horrible Rock N Roll music, not wholesome music like Guy Lombardo and Glenn Miller and the Staple Sisters.
-TV is going to rot their brains out,we are raising a generation of idiots
-What does Lionel think we are made of, money? Just look at the prices on this stuff, it is a toy, darn it.
-There would be those, of course, who grumbled about the trains that were made crappy, pieces missing, etc (yes, folks, Lionel in the 1950's had defective units, one of these days would be interesting to see reports from Lionel in the 1950's about warrantee issues.) Years ago I met some old timers who owned places that sold Lionel trains in the 50's and 60's, and they said that trains would often have issues out of the box, transformers that didn't work right. Given what I know of quality overall in that time period, fail rates might have been as high as 1 in 10 units.
-Kids are disrespectful, they would rather play with their idiot friends, they talk back to their elders and they just aren't as perfect as we were (ever read what their elders said about kids in the 'greatest generation?'....eye opening *lol*).
What adults were NOT likely thinking/saying in 1950's
-Wouldn't it be wonderful if ever house had a computer in it? (if they knew about computers, they were big hulking giants that TV and newspapers would herald in their projections of who won an election, or were the 'electronic brains' of science fiction. In science fiction of the time gee whiz computers were these things a mile long in a cavern someplace, not a small handheld computer like our smart phones).
-You know, those Lionel F3's are real crap, the rivet detail is off and is like 2" in scale, the red on the Santa Fe units is nowhere near Warbonnet Red, those couplers are the size of a rain barrel and the trains start moving at 10mph. (at that point in time, I don't think rivet counters even existed in HO gauge)
-You know, those cigarettes are a horrible thing.......
-Boy, these are great times, the kids are great, my job is great, everyone is just so nice and friendly, everyone is respectful of everyone else, the sky is always blue, it is just such a great time to be alive, everything is so perfect,politicians are just so smart, schools do a great job teaching our kids. (If you think people didn't complain about their jobs, their bosses, the politicians, the weather, bad customer service at the car dealership, etc, you also will believe WWII movies where the soldiers are all heroic, they never grumbled, called the brass various names, etc)