You're saying the same thing I did. Classic doesn't provide the cards for a good paladin deck so these cards have to come via expansions. Without expansions, a viable paladin deck doesn't exist because the classic set doesn't provide it. Same is true for priest and shaman. This means that anytime these cards aren't provided by the currently active expansions, the classes suck. All the other classes have the makings of a viable deck in their classic set:

Control warrior
Midrange hunter
Ramp druid
Miracle rogue*
Freeze mage
Zoo warlock

These are all decks that can be made with the classic set, so these classes always have something to fall back on. Rogue is a bit debatable because miracle is kinda limp without something to accelerate it, but then that also happens to be a problematic class that tends to get little from expansions and has relied on only two archetypes - miracle and oil - for the entire history of the game.

In short, the thing that all the good classes have in common is that the classic set provides all the things necessary to create a viable deck. The thing that all the poor classes have in common is that the classic set doesn't, and they rely on expansions to make them viable instead of making them even stronger.