So 100K could be poor in California, while 30K isn't poor in rural Michigan? That's the problem with trying to come up with a definition - too much varies from place to place.
Exactly. Which is the problem with your suggestion above:
To me it makes more sense to define middle class by income distribution - the statistical middle - than by subjective measures of lifestyle because lifestyle is so impacted by debt or lack thereof.
For example, our house, when we bought it, was $X. If I look on realtor.com right now, I can see that there are 44 houses in that range (more or less). There are only 16 houses (and some of those are condos or duplexes) where my in-laws live. And there is another difference - the houses locally are bigger and/or newer than the ones where the in-laws live.
That's actually one of the big problems with a national minimum wage - it presumes that the cost of living is identical everywhere. Minimum wage will buy you more (though admittedly not much) in Lincoln, NE than it will in New York NY.
FWIW, my rule of thumb is this:
I was poor when I worried all the time about food, rent, utilities, transportation, etc. I was poor when every purchase was a trade-off - if I bought generic 'nilla wafers as a treat, it meant making mac-n-cheese without milk. I was poor when a single day of missed work meant a major recalculation of the budget, and a missed paycheck could have put me on the street.
I entered into middle class when I didn't have to count pennies all the time, and I had the relative freedom to get what I wanted at the grocery store. I could afford to indulge once in a while, and even take vacations from time to time. Monetary worries are more concerned with major purchases (house, car, college, retirement) than with the mundane (Doritos, milk, a daily newspaper).
We have not yet entered what I would call "rich". Our heads are above water (though I need to reign in our spending again), but I think, when we don't have to put off a major purchase (like a car) until the financing is better, or worry about the cost of a trip to Disney. In the meantime, we have a nice house, we have 2 cars, and I have the luxury of being able to be at home for our children. (When we had kids, we worked it out - if we had to do child care, my job would have only paid about $1 an hour, after counting gas, food, work attire, etc. Wasn't worth it.)