I mentioned this in another thread, but I feel that it warrants a thread of its own.
Several people on this forum (myself included) are concerned that the Aptitude system discourages players from creating characters with unique skillsets. When advancing a character, you sometimes feel that you are "wasting" XP when you purchase an advance but don't have both the appropriate Aptitudes. The system is certainly better than the overly-restrictive DH1 class system, but it does penalize people for buying skills outside their character's specialty. Sure, my Feral IG Warrior could pick up some investigation skills to help out in non-combat encounters, but when rank 1 of Inquiry costs as much as a tier 2 combat talent it doesn't seem very worthwhile.
In my DH2 campaign, I let my players purchase skills for less XP if they spend a significant amount of in-game time learning that skill. This works for just about any skill given the correct circumstances, but it works especially well for Lore skills -- our psyker recently checked out a few books from the Remembrancers Guild (Space Library) while the party was searching for some clues, then spent a few hours each day for the next week or so studying those books. As a result, I let him purchase Scholastic Lore (Cryptology) for half the normal price. Note that I don't usually allow players to buy cheaper ability advances or talents this way for balance reasons.
Do you think something to this effect should be written into the DH2 core rulebook? DH1 had an Elite Advance system that used a similar rationale, but now that everyone can buy all skills/talents this was removed from the beta PDF.