I may change my mind when I finally get to properly use a W7 machine (given that it has a reasonable interface overhaul even vs Vista anyway), but for now... oh, grasshopper, such that you have to learn about usability and unobtrusiveness. I feel you may be in the stage I was, wayback customising 3.1 and 95 to have the largest, most garishly clashing colour, font, wallpaper and window outline schemes I could come up with just because I could (though having come from GEM, could you blame a little geek for it?).
Same as Yahtzee says about game mechanics - once you start noticing an operating system's functions, the makers have failed. It's what puts me right off MacOS. Showy and shiny is all well and good, but they've evolved it to a point where it actively gets in the way of getting stuff done, which is what your computer is for.
The (2006) XP SP3 laptop I'm writing this on? Classic theme. My workplace desk station (early 2010) with similar OS? Ditto. The default fisher-price one is both distracting, available in too few and too ugly colours (I prefer something like an understated, soothing Spruce, Slate or Aubergine, maybe with slight tweaks, whereas the only half decent-looking XP one is the default and rather garish blue), and oversized, taking up more than its fair share of real estate, particularly on a lower-resolution screen (which MS really need to be reminded are more common than ever with the rise of netbooks; none of my systems are less than 768 pixels tall, but Classic still gives a moderate advantage). The Vista-styled theme pack that we had for a while was reasonably good looking and not too intrusive (low-profile taskbars, smallish start button etc) but again - literally - came in "any colour so long as it's black"... and was buggy as hell, seeming to channel its inspiration's malevolent spirit. I uninstalled it after about a week, and as of our latest updates, it's gone from the whole site for the same reason.
Besides, it chows too much CPU/GPU time, particularly on the lower end systems that tend to get used for more serious work (if yours is a mainly gaming system, what are you doing bothering about the windows theme anyway? Alt+Enter FFS), especially if you have all that fading-menu BS turned on. I knock it back to plain jane mode (one concession: show contents whilst moving. It's not the 80s) and get a mild usability/response speed kick when heavy multitask processing is going on in the background (e.g. video coding, archiving, virus scanning and the like... especially all three + email + web + IM + noodling on something in photoshop/cooledit/office). Even on my dual core workstation. I'd hate to think how badly Aero Glass would respond under those conditions, or whatever the W7 equivalent is.
(Mind you it might actually be better - the i915 / i965 chipsets onboard those machines don't seem to be too bad at simpler 3D stuff, they just toss their cookies when faced with something like Rigs of Rods)
The windowing system and OS in general should fade largely into obscurity, particularly once you figure to get half your stuff done with keyboard shortcuts. It's a means to an end, not a main attraction in itself. A certain amount of prettification and customisability is welcome to stop it being quite so distractingly stark as yer oldskool Unix X-windows stuff (dear god, it's worse than GEM in its utilitarian bleakness... must be all that battleship grey... and the downright user-hostile (patent-skirting?) mouse/menu setup) but that's as far as it needs to go. The ideal is to show the working area as big as possible, show the essential metadata and control menus/buttons surrounding it in a clear, compact way, and keep the hell out of the way at all other times. Let the actual programs make with the flashy, if that is their wont.
This of course may have to change a bit with the coming of multitouch interfaces, mind. I'm not sure they'll catch on. This laptop is also a tablet... the (second) pen broke some time ago. I'm not too bothered about replacing it (or the failing 2nd battery), unless my work changes such that I need a fully portable, handwriting capable A4-size notation device once again. Keyboard + touchpad (or KB + five button wheel mouse, at work) are doing the job just fine over here.