The Black Dog set from preordering is probably a unique set that you can't get from in-game or microtransactions (hopefully anyhow, else it kinda takes away the point of it being a pre-order bonus).
I am expecting microtransactions to be items with a certain look.
I am expecting in-game items to be items that can only be acquired in-game (i.e. not from microtransactions at all) and that in-game items will include unique rewards based on achievements through gameplay (for example getting a pirate hat stylized to be a kraken as a reward for defeating the kraken).
I am also expecting that microtransaction items will be less unique than in-game items simply because of the fact that microtransaction items will be much more available and easy for players to buy and equip, whereas the in-game items (at least the more rare / expensive ones) would be seen less in the game due to being harder to get (requires more effort and time put into actual gameplay).
Those are my expectations at least.
I would be disappointed if I spent hours in the game and finally defeated a kraken, only to see some random I_R_Pirate_001 player rolling past me with 1 hour played and wearing the same kraken-hat that I got from spending 100 hours and losing against 20 krakens before I finally defeated the 21st one and won the hat.