my 2 cents:
-Any class that has a built in Cohort has more actions it can take per turn.
-Any class that has a spell list has access to options that classes without spell lists have.
-Any class that can Summon can generate it's own combat allies.
-Any class that can restore hitpoints has an advantage over classes that cannot restore hitpoints.
-Full BAB is better than 3/4 BAB is better than 1/2 BAB.
-Greater sized HD means greater survivability.
-More SAD is better than more MAD.
-Good saves are better than poor saves, this also means greater survivability.
-More skillpoints and in-class skills are better than less skillpoints and less in-class skills.
-More feats granted are better than not adding feats.
-Classes with class features granted more levels have more options than classes with dead levels.
-Classes with access to martial weapons or a wider weapons list have more options than classes with only simple weapons.
-Classes that can wear heavier armor will have better AC than classes with lighter or no armor, unless the class features suppliment armor for AC.
-class abilities that have longer durations, or can be used more times per day, and better than abilities with short durations or very limited number of uses per day.
Use the Commoner as the baseline. 1/2 BAB, no class abilities, 1 simple weapon, no armor, all poor saves, d4 HD, 2 skillpoints per level.
Considering all the above, Druid (most mechanically powerful built-in cohort, spell list, Summons, healing, 3/4 BAB, d8 HD, Wisdom depenant only, 2 good saves, 4 skillpoints per level, only 2 dead levels regarding class features, up to Medium armor) really numerically has more advantages than the other base classes.
Contrast to the Fighter. All Marital weapons, better BAB, better armor, combat abilites that can be used endlessly throughout the day, better HD, many more feats to choose from. This is still a number of useful features, however he has less overall, and not ones as powerful as the Druid does.
Imagine (maybe I'll do this one day) making a spreadsheet that listed these features I've described across the top, and listed classes down the side. Then, you place check marks for each class that contains the above feature. Then, you simply count which classes have the most things going for them. I'm not suggesting all of these features are the same in terms of usefulness or power, but it'd be a handy tool to use at a glance.
Can anyone think of additional, important features that should be added to my list?