Great topic.
On the delay issue -- please, absolutely look into this. I have 77 UPB switches installed and in most cases, the delay is not terribly noticable because of the ramp up (though, of course, the ramp up doesn't start until nearly 3/4 second after you press the switch).
But in places with snap on, it's a real pain. In a dark room, if I'm at a normal walking pace and press the light switch, I get between 3 and 6 feet into the room before the lights come on (with snap). You can stub yourself on quite a bit in that space.
And to echo the previous poster, the other major thing is that "did I hit the button?" worry that makes you pause and think for a second. A light switch shouldn't ever make you pause and think if you just used it. With no other physical feedback (i.e. with a normal switch, something physically moves to a new state/position), not having an instant response is a big deal.
Where I live, there are a lot of mid to higher end homes with folks looking at HA tech and because of my association with a few higher end local builders, I've been helping a number of folks out. At least half of the folks I show UPB to pass on it because of the delay. Seriously -- at least half.
These are folks who then go on to try higher end X10 switches or Insteon or in a few cases, ZWave. As folks have mentioned, all of those technologies have problem too. And these are folks who are not very "price concious" -- they'll drop 10k outfiting a home theater without blinking (often more).
And if you want more, search these forums or the Authomated Home forums or a few other HA releated forums and you will see the delay issue brought up time and time again. I'd say it really is the #1 "issue" I've seen discussed about UPB switches in the online world.
I think that if there is no double tap action, the switch should start doing whatever it's supposed to do immediatly (turn on, ramp up, snap on, or snap off). Then, ideally, any delay for double tapping would be configurable. For folks in houses with nimble fingers, they may set want to set the double tap delay to 1/4 second, but for folks who are older/arthritic or disabled, they might want to push it up to 2 seconds or so. Making adjustable means everyone gets the rate they want (vs a "one size fits all" approach that rarely fits all).
Having spent time with most of the technology out there right now, I really feel UPB is the best wireless/refit technology out there. The cost is a bit higher, though when you start equiping a house and buying 20 or more units at a time, you can get a significant price break, so that they aren't much more expensive than ZWave and are cheaper (in quantity) than some of the highe end Leviton X10 switches. The switches are incredibly flexible and the UPStart software gives pretty good control over things (see notes below though).
The light bars things others have mentioned would be nice, though it's never come up in my conversations with folks. Putting a full RGB led for the pilot light would be very cool to allow any color to be used (but I'm a bit off on that stuff -- see rgbled.org).
One last issue -- UPStart. It's frustrating to use sometimes. It seems every vendor has different options enabled or disabled and so if you download it from SAI, then certain options (like graphical editing) aren't available for HAI switches or PCS switches, etc. There seems to be some (perhaps unintentional) attemp at lock-in and that doesn't sit well with me. Also, some new tools to let you do things like copying/duplicating existing switches and links (especially links) would be a huge thing. I often want to make a new link that is just like an old link but for a few minor differences. If the link has 40 switches, right now, since there is no way to copy that link as a starting point, I have to manually re-type those 40 switches into the new link. Phui. Being able to just copy a link and then edit a few switches would be a huge help.
Finally, it would be a great help for editing links to have some sort of spreadsheet mode where you could see all the devices in the link and their levels and faderates all on one "spreadhseet" and be able to edit them there (vs doinking around with each and every switch -- another tedious process).
I don't mean to kvetch because I really do like UPB. I also realize that not all of this may be fitting into what you were looking for with this topic. Please forgive the length, but given whose ear is listening, even if just a little, I couldn't pass up the oppotunity to share items from my UPB "wishlist".
Gerry