Best: Buffets - It makes life so much easier with food allergies/sensitivities/etc.
Worst: I guess for me it's general categories of experiences..
1) Guests treated differently: If you're ordering food for your guests, do not feed some one thing and others another unless it's a prechosen option on the RSVP. There is nothing worse as a guest than seeing one guest served something delish, then have something else plunked down in front of you (think unfilled short slice sheet cake vs. tall filled multi-layer cake)
2) Not enough food! Yes, some guests may not want cake, but others will be happy to have four slices, and there's NOTHING worse than after having the heavily garlic laden meal to be told "sorry, there isn't enough cake!" and the person next to you took a bite and said "oh I'm full!" Or one time the couple did stations - only the caterer ran out of food about 80-100 guests from the end because people were eating more because their ceremony was during lunch and this was 7:00 at night with little time to stop to grab a bite ahead of time - take a guess how long people stuck around there!