Really good food, not the cafeteria/airplane food that people have come to expect at wedding receptions. Tip: doing the meal "family style" is a great way to save money on catering AND get everyone at the table interacting.
Lots of booze (no cash bar, if budget allows!). Throw in a signature cocktail that has meaning to you to add a personal touch.
Short, fun speeches. Make sure you edit your toast-makers, and encourage them to throw in some amusing, light-hearted commentary. If you choose to honor a deceased relative(s), try to do this silently during ceremony (with a mention in the program or by lighting a symbolic candle in their honor) rather than during the reception -- the reception is all about fun! Avoid slideshows unless they're no more than 3-5 slides and well-produced - you don't want guests to feel like they're at a lecture or meeting!
Great music. It can be a live band or an iPod, just make sure the tunes are rockin'. While it may feel cheesy, play songs that people are familiar with (remember to play so oldies-but-goodies to get the older generation on their toes). Now is NOT the time to demonstrate your in-depth knowledge of obscure electronica!
Thoughtful assigned seating. While this depends on your crowd, it is often stressful for guests to have to figure out where to sit if seating is not assigned. At least assign guests to a table, and give thoughtful consideration to seating people together who will have something to talk about (this takes time and is often left to the last minute -- so start early!)
Fun favors and ice breakers. Avoid impersonal and/or cheap favors like a trinket with your name engraved in it. Instead, do something fun and interactive, assuming you're not having a super formal and serious wedding. Ideas include mad libs about the wedding or couple, song request cards (including songs they don't want to hear!), fortunes, jokes, parlor games, bingo (guests can play based on certain words or phrases they hear uttered during speeches), etc. Fun novelties (e.g. stick-on-mustaches) or gag gifts are great favors, too
Favors for kids. Toddlers screaming during speeches + stressed-out parents = reception downer. If kids are invited to your wedding, have special favors at their place settings: inexpensive toys, puzzles, games, lollipops, etc.
Transportation. Provide shuttles back to hotels (starting early for older folks and running late for younger ones) so guests can let loose and have a good time.
Usually, the reception is preceded by a cocktail hour during which the guests get warmed up for a lovely reception:
Don't let the cocktail hour go more than about 1-1.5 hours -- otherwise, you'll end up with guests who are bored and/or wasted!
Have interactive activities for guests (other than drinking, of course!) -- a photo booth, guest book, etc. One of our favorite clients made seating cards that looked like library cards and had both the guest's name and "cross-references" on each indicating other guests with which the guest might have something in common. This was labor-intensive, but an awesome conversation-starter.
Personal touches. Take the time to include some personal or hand-crafted touches to your wedding, including personal thanks and shout-outs to people who have contributed or traveled from afar to share in the celebration. The more personal touches you can add, the more guests will feel connected, privileged to be sharing this special day with you, and genuinely want to celebrate (see our book, Handmade Weddings http://www.hellolucky.com/handma... for ideas). If you have talented friends or family, call on them to share their skills in creating the reception details or providing entertainment.
Have fun yourself. If you're having fun, your guests will follow your lead. With this in mind, be sure you have reliable folks (wedding planner or equivalent) helping you coordinate the reception so the timing and flow of guests runs like clockwork. Guests should never be at a loss for what to do or where to go -- you absolutely need someone (not you!) directing the show behind the scenes. That way, you can relax and enjoy the party!