as for why des and penny would be connected beyond sayid and nadia, at least in an island sense... penny continued to haunt and hunt for desmond while he was on the island, with her being desmond's sole purpose for moving on. sayid eventually got over nadia and fell for shannon.
Sayid knew Shannon for something like two weeks in island time. He pined for Nadia before getting to the island. He pined for Nadia while on the island and, later, married her after the Oceanic Six escaped. She didn't die until sometime after that. Sayid was so devastated he took Ben up on his offer to become Ben's personal assassin in the war against Widdemore.
With all that, we're supposed to believe his constant/soul mate was Shannon and not Nadia?
again, i think it was an island thing. ...
Then we're back to the Desmond/Penny situation. Shouldn't Des's constant been, I dunno, Clancy Brown if that's the case?
is that really such a huge sticking point either way?
To the extent that a story builds up a mythology/scenario/rules and then doesn't consistently follow them (in various incidences), yes. this is one example but not the only example of the writers seemingly not keeping track of their own "rules."
And if you're going to set up the final season that the entire series is not about sci-fi but about a series of magical rules you ought to at least try to follow them.
