I'm going to disagree (slightly) and say theme-ing is at least an equal partner to quality&quantity of rides in bringing in the average park-goer. Although the former (theme-ing) may not be play as overt a part in the average park-goer's mind ("where's the beef?"), I think, consciously or subconsciously, it at least equals rides.
The above is based on the only empirical data we have: attendance. The well-themed parks leave the lesser themed ones (even within the Disney roster) in the dust. Like the average park-goer, I'm a ride guy, so I agree with you that parks like the Studios and DAK need more rides, stat, to be worthwhile and fulfilling. But I'm also compelled by their inbetween theme-ing (particularly DAK) as much as by rides. And a well-themed but attraction-lite park (DAK), routinely outdraws IOA and USF by a significant margin. While there are reasons for this in addition to theme-ing, this fact doesn't support the "average people prefer rides over theme-ing" theory. Granted the parks with the fewest rides (DAK, MGM) are the least visited among the 4 WDW parks (and again there are reasons for this in addition to # of rides), so I'm not understating the importance of rides, just saying that theme-ing plays as important a role.
To your point, look at HKDL and its direct competitor, Hong Kong Ocean Park. HKDL is well-themed but very light on rides. Ocean Park is closer to Busch Gardens/Six Flags in its level of execution, but has many more rides and has outdrawn HKDL over the last five years (again, there are other contributing factors). However, HKDL is closing the gap, and by 2013, with its three new areas (and 5 new rides (3 of which are kiddie rides), Disney, while still having significantly fewer overall rides, will likely outdraw Ocean Park. If that happens, it will show that success in theme parks is the one-two punch of rides (attractions) and great theme-ing.
Bottom line: like popcorn movies, theme parks are popular because they provide escapism. The best parks (and most popular) provide a total experience, where the illusion is maintained as strongly as possible. Disney has historically been better at maintaining the illusion (theme-ing) throughout the park than Universal or anyone else. Seeing a giant Potter showbuilding from the Jurassic Park jungle or walking through a medieval castle only to board a modern steel coaster are bubble-bursting design choices - the escapist effect is weakened and people aren't as compelled to return: attendance figures seem to reflect this.
I still think IOA is a top tier park in both attractions and theme-ing. While it makes some sacrifices in theme (e.g., naked coasters), it makes up for that in high-octane rides and lot of its theme-ing is on par with the best of Disney. It also opened with a full day's worth of attractions, which is pretty anomalous.
From its design, I think (wish) IOA should have been a home-run from day one, upping the ante for WDW and WDI. Sadly, it wasn't (for a number of reasons: botched marketing/name, market saturation, subject matter, etc.) and has limped along for a decade, allowing WDW to rest on its laurels. Like most people, I think Potter is fantastic and hopefully heats up park development for both Disney & Universal. Eagerly awaiting the 2010 attendance report.