In theory, you're loosing on some, possibly minor, stuff.
1) If you're main character is an origin one, there are some lines in dialogues you couldn't choose otherwise (i.e. as Ifan you can entirely avoid some confrontations, which wouldn't be possible otherwise).
2) Some solutions/options for origin character's quests are available only if you play as those characters, and you are able to unlock related achievements only when playing as them.
3) As for what CatR wrote about achievements, they are bugged in both Steam and GOG versions.
