I cant believe some people convince themselves that they like aqua. It was fine 10 yrs ago. Not anymore.
Hard to accept different opinions, eh? I too like Aqua, because it is _different_! It is not the usual interface, but instead brings some fun into computing. Think of the "Mac and PC" ad campaign: Aqua is the Mac, grey-in-dull-grey interfaces are the PC.
I still like the Cube and its matching TFT displays, partly also because of that pinstripe look, which makes a great match with Aqua. Since Apple slowly, but surely migrated to a more "serious" look on both machines (Aluminum and black instead of Acryl and white) and interface, in my view they also lost the "WOW"-factor somehow. Back in those days they stood out with machines that _exhibited_ the "Think Different" mentality. Think of the coloured iMac G3, "flowerpot" iMac G4, Cube...
You could really see the fun the engineers had in tinkering with new approaches. Computers suddenly were "emotional" again - people bought because they felt attracted on a non-rational level. The Amiga had a similar effect - people had some emotional relation to their machines with computer chips having real names (like Agnus, Denise, Paula etc.) instead of cryptic numbers or functional names like memory controller or Audio chip.
Of course you have to do evolutionary steps - but every now and then it's time for something revolutionary. If you don't do that and only make evolutionary steps all the time while trying to become more mainstreamish (and going for rational decisions only), you suddenly find yourself within heavy competition. If the machines look pretty much similar to what is already there in the market, people start to compare. Why pay a premium for an Apple box, if the OS is the major distinction to a Wintel box, which looks pretty similar? That's why many people go for a hackintosh these days.
And if Apple now slowly mutates its interface to be (even more) mainstreamish, people will start to ask, why to even bother with struggling with all the hackintosh issues, when they could go for Windows 7, offering a similar experience (at least in their perception). People might argue now, that OSX still has some unique features. But the question is - which of those are really necessary and recognized as such by the user? And which are offered in only slightly different form by other OS's like Windows or Linux as well?!
IMO Apple seriously has to rethink its business model. Until now, they targeted the design-oriented geeks, designers and other people feeling and/or behaving different. They were ready to pay the premium Apple asked, because they were getting a really different product and user experience and - most important - made their purchase decision more on an emotional rather than rational basis.
If Apple decides they have to pursue high numbers of machines by going to look and feel mainstream, they'll lose much of the emotional aspect (often referred to as Steve Jobs' Reality distortion field

and should better be prepared for intense competition (i.e. price as the dominating factor). If people start to base their purchase decision solely on rational facts (which is fostered by going to look more and more like everyone else), Apples behaviour as a company (e.g. no roadmaps) together with their huge price premium may in the long run have a negative effect on their sales.
Back on topic: People feel Aqua is outdated - maybe because it has not been really updated ever since OSX 10.1. I think the dynamic "waterish" effect could be improved. Make it more realistic, for example. The progress bar could have slower moving "waves" with irregular distances instead of the current uniform waves which have exactly the same distance to predecessor and successor elements. Make it visually roll around the longitudinal axis or irregularly sway back and forth within the bar. Add a very short ripple effect when moving windows and icons (and make this optional so users can switch it off if they don't like it). Use "water drops" in Spotlight instead of (or together with) light circles to highlight search results. Make menus "flow down" instead of simply opening in an instant.
You want to better match the actual silver/black look of the current machines? Give the scroll and progress bars (animated) water in black! Bring on white-on-black pinstripes (isn't that what all the serious businessmen are wearing anyway?) to the window frames. Just don't forget to continue to offer the original Aqua theme for people like me who still like its colourful, yet non-obstrusive look even in aluminum machines (by far more harmonic than e.g. Windows XP's candy look).
What else? I'm sure other people could easily dozens of other suggestions how to really update the Aqua theme without getting too serious and "business-like" by going towards granite, marble, brushed metal and whatever else looks greyish.
Maybe Apple will still surprise me with a really realistic black-and-white marble look that is matched by applied foils giving the computer a look as if its housing would be made of real marble. Something similarly overwhelming and "different" as the first series of coloured iMacs. But seeing the development over the recent years, my hopes are not exactly high...
/Rant over - thanks for reading.