Location, Location, Location
Also, just like with weddings, your location is a key factor in your event. One of the most important determiners for choosing your location is the date of your event. Many popular event venues book up months -- even years -- ahead of time, so you really can't pick a date until you're sure you've secured a venue.
The size of your venue will have maximum capacity, so it will also dictate how many guests you can accommodate. Also consider matters like parking and having ample space for dinner service and entertainment.