I remember a while ago, J. Michael Straczynski was asking if anyone knew how to contact him. While JMS kept the reason why to himself, many speculated it was due to Michael O'Hare's declining health.
Sad to hear of his passing. I really liked his portrayal of Commander Sinclair and will defend his acting in Babylon 5 every time. Sinclair was supposed to be played as somewhat wooden and Sinclair captured that.
Now that I think about it, his portrayal was a much quieter and stoic one, similar to Jeffrey Hunter (both of whom, of course, were replaced with much more lively and energetic performances).
Admttedly, I think I preferred Bruce Boxleitner, but I've sometimes been curious how the series would've played out if they'd stayed with Sinclair the whole way through. I know that he would've been the one who hooked up with Delenn, and Carolyn Sykes and later Catherine Sakai would've assumed the role of the love interest who is presumed dead on an expedition, only to later return alive as a brainwashed Shadow agent (a role that was filled by Anna Sheridan).
Anyway, farewell, Minbari not born of Minbari. He's gone beyond the rim to reuinite with Dr. Franklin, Zathras, Dr. Kyle, G'Kar, and Zack.