Casing
I'll be the first to admit - the casing is butt-ugly. It also wouldn't be much fun to fall on. It's not particularly aero, if that matters to you.
It is very lightweight, however, and this is hard to maintain with a nicer casing. Suzy Jackson's light has a very nice casing, but it looks heavy, and I also lack access to a lathe with which to build such a thing. PVC tubing is another option, though heat may become an issue (unless you leave the front and back of the case open, and have a heatsink in the middle to catch the air rushing through).
I'm pleased with the reflector mount - it seems more than strong enough. The only thing that I'd change is to make it easier to mount and unmount the light without a screwdriver. This may be possible with a commercial light bracket.
Batteries
The batteries have served me extremely well. I love the modern NiMH AA cells; they store a lot of energy, are very cheap, and are not even particularly heavy. Commercial fast chargers are cheap and reliable.
The main improvement here would be convenience. It's a pain to remove the holder from the bike, remove the cells from the holder, plug them into the charger, and reverse the process later. If I had commercial battery sticks that clip to the frame like a pump, that would be far more convenient. I've never purchased commercial lights, so I didn't have any motivation to keep compatibility there.
Future revs of the battery system will probably use stick-compatible casing and a DC socket so they can be easily removed and charged. This adds a lot of hassle during construction, since I also have to build a charger. One big advantage of the frame-mounted batteries is that I don't have to install high-current cable all over the bike.
A bottle battery is another option, but it uses a whole bottle cage, which is an issue for long-distance riding. My road bike has nowhere good to put a third bottle cage (and if it did, I'd probably want it for another water bottle). I did take the bottle battery option for my mountain bike lights (coming soon!)
The li-ion battery packs for my laptop are quite cheap to replace (AUD$70) and extremely good capacity (66Whr in 400 grams), so it'd be nice to be able to use them. They're somewhat unwieldy to mount to the bike. Alternately, I could get the 18650 cells off eBay and build a charger. With the li-ion cells, I'd have an extremely good weight to power/battery life ratio.
LEDs
I'd like more light, but experiments with some alternate brands of LEDs and lenses have been disappointing. I'll probably stick with the Lumileds emitters and Fraen lenses, but go for the new Luxeon K2 variety for better efficiency. I might also install a second emitter/lens pair - they add very little additional weight.
The use of a resistor as regulator is somewhat dumb. It burns off about 20% of the power used by the whole circuit. So far, battery life hasn't been an issue for me - but the engineer part of my brain just won't let it go. A proper regulator like a Buckpuck or a Boostpuck would reduce this wastage to about 5% and make it possible to use any old batteries that happen to have a compatible plug on the end.
Miscellaneous
A power switch would be nice. From my mountain bike lights, I've also found a 'dim' setting to be useful, especially when moving slowly or sitting at traffic lights.
Heavier cabling and better reinforcing on the battery pack would make me less nervous, but it's held up fine so far.
The future
So my next road bike lighting system will probably look something like:
Some of these have already been addressed in my mountain bike lights - check back for more information!