Here's the deal as best I can figure it.
A list games: Games that are classic, hold both their value and popularity indefinitely. Mind you, an A list game does not guarantee that you will like it once you get it as many pins just don't appeal to the casual player. But an A list game pretty much ensures that you will be able to trade it towards another A list title that you might. And an A list game usually goes for a minimum of 1500 but most are 2800+++ if you know what I mean.
B list games: Are the other 70-90 percent of pins out there. They are good fun pinballs that most regular collectors end up with. Prices are usually 800-1500 depending on the titles. They are higher production numbers and often are made by the less popular companies like Data East, Stern, Sega and include older Bally/Williams like system 11 and older. They are good pins, and affordable, and usually within the regular collectors grasp.
C and D list games: Mostly, I would think this would be more a statement about the condition of the pin itself. You could have a B list title with horrible play wear, missing art/playfield parts, bullet holes in the side of the cabinet....You get the idea. Its not impossible to restore or repair, but its not going to be cost effective and the battle will be uphill all the way. That's not to say its impossible to find one of these pins that's both playable and fun. Maybe its just missing the back glass, but the rest of the game is fine. But then again, who would want to look at a light box blinding you while your trying to play. Then again, maybe its just a filler like an old pre system 11 pin, or even a bingo pin or "bad" EM pin. Not wanting to get in an argument over how awesome "some" EM pins are. I am sure they are some great ones, but I am equally sure that some are slow and boring, and simplistic ones that are probably a pain in the arse to maintain. Think 300 bucks for an average price.
I do believe that there are probably some exceptions to these generalizations. Some DMD pins that are just bad enough that most collectors would not like to own them. The one that comes to my mind personally is "Dale Jr." and maybe "Nascar" by extension. And from what I have heard, Transformers may end up on this list if they don't fix the programming bugs. Not played it myself, but I have heard its lacking. But it will still probably keep its value. Shaq Attack maybe fits the bill. Look up Gottlieb street level pins. They would probably have a few titles in the D list.
That's not to say a D list pin can't find a home. But it could be better to part out if its truly in rough shape.