Boy that's a good question. I would have said "battery life improvement" before the new batts came out...but they have largely solved the problem for me. I wouldn't mind some kind of built-in top screen...maybe a slightly smaller form factor...a little more consistency from unit to unit...but overall I have few complaints. Mostly, I trust the FF guys design-wise, and I'm really curious to see what they have in store for the FF 2. I know it's coming, I just don't know when, or what it'll be like. The mystery is pretty sexy at the moment.
I didn't try the older batteries, so I can't comment. The batteries on my new unit are perfectly fine. Of course, more capacity is always better, but, no complaints from me either. Size wise, a bit smaller or lighter wouldn't hurt, but, I'm not using this for maximum stealth. The physics of size versus functionality has limits. The Pax, for example, is wonderfully small and sleek, however, function (e.g., flavor, heat control, ease of cleaning) is sacrificed.
My wish list:
1. A battery meter. It would be nice to know how much power remains.
2. Eliminate the need for screens or other wear items (besides the battery). I don't mind the lack of a top screen. It's one less thing to replace. Perhaps the current design can be tinkered with to reduce herb traveling into the vapor path. This hasn't been an issue for me. I don't like the screen at the bottom of the bowl. It's an inelegant part of an otherwise wonderfully conceived device that doesn't belong in terms of design or function. Really, it's a pain to remove and replace. When I tried, I bent and damaged it. Putting in a new screen by following the recommendation of creating a dome enabled me to fit it in, but, the dome didn't completely flatten and now more herb gets under the screen than before.
Here's my idea.... Keep the existing hole pattern on the bottom of the bowl (obviously necessary for air flow), but, have a tube leading from the bottom of the bowl (on the inside of the device) to a clean out on the under side which can be opened to remove any herb that falls through the holes. With enough small holes in this tube, air flow will remain sufficient without enabling herb to enter the inside of the device. It would eliminate the need for a screen and thus would be a more elegant solution for maintaining the Firefly.