Wise men regret that "their words had forked no lightning," perhaps that their words never achieved the power the men wished them to. Since lightning is such a powerful force that creates a huge impact, it seems likely that these men regret the lack of impact they or their wisdom has had during their lifetimes.
Good men, with "the last wave by, [cry] how bright / Their frail deeds might have danced in a green...
Wise men regret that "their words had forked no lightning," perhaps that their words never achieved the power the men wished them to. Since lightning is such a powerful force that creates a huge impact, it seems likely that these men regret the lack of impact they or their wisdom has had during their lifetimes.
Good men, with "the last wave by, [cry] how bright / Their frail deeds might have danced in a green bay." It sounds as though the waves were their opportunities to do good in the world, and, with their last wave having gone by, these men recognize that they will have no more opportunity. Further, they feel that they might have done more good if they had only been stronger.
"Wild men who caught and sang the sun in flight, / [...] learn, too late, they grieved it on its way [...]." Perhaps they thought that they were living a full life, living without regret or without wasting any time, but then they realize that they were really mourning the passage of time all along. In the end, they regret having done so because it prevented them from living as full a life as possible.
Grave men who "near death [...] see with blinding sight / Blind eyes could blaze like meteors and be gay [...]." Serious men, then, as they approach death, suddenly realize that it was possible for them to be happier even as they aged. They regret not taking advantage of the chance to be happy when they could.