The Question I find myself puzzling is whether or not Ichigo is actually Isshin's biological son?
It's pertinent because Isshin said, " The thing is you're not really a Shinigami..But you aren't a mere human either. Your mother was.... A Quincy".
The phrase "you're not really a Shinigami", What does it mean? Ichigo should be equal parts Shinigami and Qincy, shouldn't he? Unless if he weren't Isshin's biological Son. I agree that depending on how one parses the phrase, one could say "Not Really" means Only Partly.
So that Rukia's power transfer kick started his Shinigami half, leaving the Quincy half dormant. Also let's not forget Aizen, and maybe even Squad Zero may have been playing Cupid.