1) Attend to guests' most basic physiological needs:
Drink: Keep the bar free, stocked, and open. Enough said.
Food: Have "normal" food available for those who don't share your love of smoked eel tartare. Also make sure to have options for vegetarians and those with food restrictions. If you want to include some "out there" food choices, do it during the cocktail hour.
Privacy: Always check the capacity and condition of your site's bathrooms. If you're renting portable restrooms, spring for the "upscale" models (your guests don't need a Lollapalooza flashback).
Safety: If you're hosting an outdoor ceremony or reception, you need to protect guests from the elements. Stock up on bug spray and sunscreen, serve chilled drinks if it's hot, hand out inexpensive wraps if it's chilly, or supply umbrellas for rain. Use nonmetal chairs and sun shields if it's sunny. And get a sound system with microphones so everyone can hear your vows over the wind. As for post-wedding safety, have a list of local car services for guests who've been hitting the bottle too hard.
Family: If you're inviting children, have a plan to keep them entertained so that they—and the adults around them—can enjoy the party. If your wedding is kid-free, have a list of reputable babysitters to offer parents. Your oldest guests have special needs, too. Schedule traditional wedding events like the first dance and cake-cutting early; make sure there are seats during cocktails, and seat these folks far enough away from the music speakers but close enough to feel a part of the festivities.
2) If you really want to score points, fulfill your guests' need for fun:
Social inclusion: Guests almost always prefer some direction, so don't force them to endure a high-school-cafeteria style, "Where should I sit!?" crisis by skipping escort cards or a seating chart.
Attention: For out-of-town guests, welcome notes are a must. If you have the budget, also include bottled water, a snack, a map, an itinerary, and a list of recommended activities and restaurants in the area.
Activity: If you schedule a long gap between the ceremony and the cocktail hour, guests are pretty much guaranteed to wander off and/or engage in recreational boozing. Avoid this situation or plan sanctioned activities to fill the downtime.
Respect: Guests don't like to wait, especially when they're hungry. Make sure you've supplied enough food and servers to avoid long waits, and offer at least one stationary hors d'oeuvres spread. Waiters passing drinks when guests first arrive will prevent huge lines at the bar. Also, make sure the coat check doesn't create a logjam—one attendant per 100 guests is recommended.
Entertainment: Your band or DJ can read a crowd, so let them. Never shut down a full dance floor—even if it means postponing the cake-cutting. As for toasts, limit the number of speakers, and ask them to keep their speeches to two minutes (five at the most). Stagger the toasts between dinner courses to avoid a block of (potentially boring) speeches.
3) For even more guest enjoyment, create a respectful, orderly world:
Security: Everyone needs to know where to be and when—'but it's safe to assume that everyone has lost the invitation. To get guests from the ceremony to the reception site, hand out time/direction cards at their hotels and the ceremony venue. If you have the money, charter a bus for ceremony-to-reception transfers.
Lodging: Out-of-town guests need a convenient and affordable place to stay, so reserve a discounted block of rooms on their behalf. Suggest at least three hotels in different price ranges on your wedding website.
Clothing: Advising guests on what to wear on the wedding invites is helpful as long as you keep it general, like "black tie" or "cocktail attire." Don't make them sacrifice their comfort (emotional, financial, or physical) by demanding they dress entirely in pink or requesting they adhere to an indecipherable theme like "Glamorous Steampunk."