Re Personalities: What they said - but even more so.
He says he is about to tell us essentially 'What is the only thing that really matters', and then he tells us. Here the single word 'Personalities' carries the weight of a whole sentence. In the one word he is effectively saying - "The only thing of real importance is 'Personalities' ". To not capitalise it is to risk the reader mentally 'rushing on' past the 'sentence condensed into a word', and then having to retrace their mental footsteps to understand his point.
Whitman is master enough of the language that he could easily have expressed himself far more clearly. Often enough his style is to use a degree of obfuscation, to not let the reader forget his degree of erudition and to put things in such a manner that the putting is, if not of as great import as the content, at least a significant part of what he hopes you will notice. When one achieves a certain level of acceptance such constructs are seen as masterly, impressive, and 'deep'. Lesser mortals, especially 21st century ones, attempting such complex styles are more likely to be seem simply as pretentious.