I still think the worse pre-order sin is actually during a game pre-order's aftermath when the game's already out...
Okay... So, let's pretend that you did not pre-order Game PO, despite the pre-order bonuses ranged from exclusive skins to story-related content, and just decide to just by the "vanilla" version of the game after the price was low enough to get it... (I mean, the reviews have been positive for the most part... Not too positive, per se, but enough to still make you go "I'll buy that game after the price is so low, it's like I'm buying it used on Amazon/E-bay"...) You boot up the game and as it boots up, it reminds you about those pre-order bonuses you missed out on by always checking to see if you've downloaded any DLC... Now, at first, this doesn't bother you... You can easily ignore it, since it doesn't even last long enough to dwell on that fact for even a good long second... However, as you're playing the "vanilla" version of the game, the game keeps hammering on about the content you DON'T have because that's all DLC-related... and the worse part about it is that these constant reminders about the lack of DLC you have all relate back to the pre-order bonuses that were exclusive to just being pre-order bonuses... You can't buy them separately for X amount of digital cash/bones, yet the game will keep pestering you about said bonuses in the form of DLC...
But wait... It gets even better! The GOTY-type version of Game PO, also, does not have those pre-order bonuses in the form of DLC... and the game will still reminds you about said bonuses since (surprise surprise) that was the true way of experiencing a game such as this... In other words, not only was content held away through a pre-order bonus, but legally you can't get said bonuses in order to have the "full experience" the game was suppose to have offer... and not even the GOTY-type version helps in this situation...
So, that's what I think is the worse pre-order sin, since it's something that not even the consumer can prevent from happening in the long run...