Le grand question

Weddings are difficult. The perfect time, the planning, the dress, etc. - all of it is tough, but perhaps no single wedding issue is as prickly as the guest list. (Although my parents and in-laws get along - for those whose parents don't, I suppose anything can arise as the prickliest issue.) I don't know if there's a legit real answer to "Who do I invite?" so all I can offer is, as always, my opinion on the matter. Of course, if you don't have a limit (how nice for you), you can always just err on the safe side, but for those of us given a max, here's how I've arranged it in order of priority:

Priority 1: The absolute musts of the guest list - bridesmaids, groomsmen, their husbands/wives/fiance(e)s, eidim, and couples you are unquestionably still friends with who invited both of you to their wedding whether or not you were engaged at the time.

Priority 2: It's more like priority 1b - the obvious good friends, whoever your roommate was when you got engaged, serious boy/girlfriends of your groomsmen and bridesmaids (no, you don't have to, but it's a nice way to show your appreciation for everything they do for you, and also make them not hate you), couples you are unquestionably still friends with who invited one of you.

Priority 3: Any friend whose presence you know will enhance your wedding in some way (think old high school friends you don't really talk to anymore but love seeing, old chavrusas, friends who are generally mesameach chatan v'kallah, people who have generally spent a lot of time with you as a couple), boy/girlfriends of good friends whose weddings you're pretty sure you'll end up at, anyone you lived with while you were dating your current fiance(e), and yeah, sometimes you just invite someone because you invited every single one of their friends. It's not a must, but if you've got the space, it can't hurt. If you don't, don't stress - how much fun are they really gonna have at your wedding anyway?

Also, admittedly, I let certain things influence my decisions. Calling when we got engaged was pretty key, or at least texting/IMing/facebook posting. Coming to our engagement party was a major bonus. In general, it's just nice when people care that you got engaged, which sort of translates into "our union is meaningful to you," which in turn translates into "you care that we're getting married," followed by "since you care more than others obviously do, I will invite you and not those others."
Probably the biggest question about inviting is "Do I have to invite everyone who invited me?" I used to think that the answer was a resounding YES, but I'm over it. Frankly, there are some people I haven't spoken to since their weddings, which means that at no point in the 9 months I've been engaged have they even acknowledged it. Does that make them bad people? Not at all. But do I care if they're at my wedding? Not at all.
Oh, and to address another question I've been asked - no, you may not split up an engaged or married couple, even if you think your friend chose really, really poorly. You will never find anyone who thinks this is acceptable.
*Note: On the other end, if you're invited as a couple and your significant other has no interest in going, don't force them unless you really don't have friends. They won't have fun, and you're forcing someone to pay a lot of money for them even though no one actually cares if they're there but you.

Major nota bene: The head count can be changed up until a couple of days before the wedding, so ALWAYS let the couple of know if for some reason you can't make it to the wedding. Barring a major emergency or the like, there's almost no faux pas worse than responding yes to a wedding and simply not showing up.

Oh yeah, and RSVP!