There is, unfortunately, little in the form of hidden benefits that you would likely miss, when you reach gold which is very much an intermediary level. You get all the traditional priority services (search "sky priority" for full description) and lounge access (with one guest if you want). You get one extra piece of checked luggage on all fares. You will get extra inventory in Y award tickets but that appears automatically when you make a search anyway, and the same with access to preferred seats which again will show automatically when you assign seats as long as your FB card is in the booking. You also get "economy comfort" seats for free on DL domestic flights only. On KL flights and DL long haul flights you get it for a discount and in fact KL will sell you many paid options they sell to others with a 50% discount instead of the full price.

There is also a list of benefits with partners (e.g. Avis, some hotel chains etc) which are comprehensively described in the FB pages (go to the section on partners) but those are (1) few and far between in principle, (2) often not granted in practice as most of the hotel/car companies have never heard of them and begrudge the perks.