For the past few years, I've been keeping an eye on the booking options within the WordPress universe and, frankly, I am astonished that still, in 2011, there aren't any realistic products.
The few that do exist have no thought or style behind them, are poorly supported, ridiculously inflexible, rarely updated, insanely priced and foist horrible design and formatting decisions upon you.
The net effect it that there are no real booking options on WordPress and the received wisdom is that, if you want that sort of functionality, you've got to go and get your hands dirty with Joomla or Drupal.
I don't see any technical reason why booking functionality should not be just as possible on WordPress, I suspect the problem is that WordPress developers have traditionally had blind-spot for this huge market, they simply don't yet get how massive the demand is, despite the never-ending stream of people searching and asking about it on forums.
What we are talking about is the ability for the user to:
1. View a calendar showing what dates are currently available.
2. Choose a date or range of dates.
3. Pay for that slot.
4. Have those dates immediately become unavailable to others.
5. Receive an email notification.
... no other bells, whistles or fancy formatting required, leave it entirely up to the post to describe and depict what that bookable item is, just give us that bookability.
It doesn't matter if we are talking about babysitters, photographers, language lessons via Skype, vacation apartments, hookers or xbox rentals; it doesn't matter if you want the user to pay a 10% deposit or the full amount; it doesn't matter if you have one bookable item or thousands ... a plugin that can handle those five basic steps will be universally useful.
It is as if we are in the stone age of the Internet - how many millions of man hours are wasted every year as customers submit enquiry forms and wait for businesses to check their schedules and reply by hand? How many transactions end up not happening simply because we've got humans doing what it would be trivial for computers to do? It's crazy!
If you think about it, this would be the perfect extension for what Gravity Forms already does and I know that RockGenius would implement it well and, unlike existing "solutions", would keep it simple, allowing WordPress to handle the content side of the equation, allowing us to decide what damn photo-slider we want to use.
I truly believe that no other Gravity Forms feature could have as much impact. At a stroke, it would pull the (extremely lucrative) market for bookings, appointments, reservations and rentals right out from under the existing systems that people are forced to use and bring that entire sector over to Gravity Forms and WordPress.
And how many new forms of commerce and interaction will spring up if you put the power of running a simple booking system into the hands of regular WordPress users, easier and cheaper than it has ever been before?