Hello John, hello everybody!
I found this thread a few days ago and finally had the time to build it and try it (on testnet). I'm trying to dump all my thoughts about it here ... sorry for being so unorganized.
1.) John & Co: thanks for doing this. I think the bitcoin-community is now huge enough to deserve a better GUI

2.) Even though I'm a Gnome user I think Qt is the right choice for the GUI, it's relatively high-level and allows to write readable code. And the platfom-support is very good.
3.) I'm not that experienced with build-systems but my colleages advised me to only use autotools if it's really necessary or you are already fluent in using them, otherwise there are less frustrating options, cmake was mentioned explicitly; autotools was called obsolete. Maybe we should ask the core developers, why they want to use autotools and if they considered alternatives.
4.) Like others here, I also welcome the removal of this irritating "Your bitcoin address:" section, the solution here is better. But I think it's still suboptimal, because of the redundancy between address book and receiving addresses. My proposal: remove "receiving addresses" and add "Receive coins" right next to "Send coins". When the user clicks "Receive coins" you show a pop-up saying [1] "Your wallet may have an unlimited amount of receiving addresses. It's recommended to hand out a different address to everyone sending you coins, because the address is usually the only way to determine who sent you the coins. You might even want to create a new address for every transaction. Addresses are never deleted, although you may delete it from the address book, if you don't plan to use it anymore. If you later receive coins to a hidden address it will show again in the address book automatically.". Below is a checkbox "Don't show again" and an OK-button which brings you right to the receiving tab of the address book.
5.) Like mentioned in 4.) I'd like to have the delete button activated with a tooltip "Delete the selected address from the address book. You will still be able to receive coins to this address". It's clear to me that this new workflow will need a change in the backend, namely having the possibility to set an address record as "hidden" in the database. But IMHO this is worth the change.
6.) I think the "Set as default receiving address" is obsolete now? If not, please tell me what it does. (And it probably should be disabled by default).
7.) Dialog "Edit receiving address": Address shouldn't be a text field if you can't actually change it? Not sure about that though.
8.) Dialog Address Book, Tab Sending:
a.) "Copy to Clipboard", "Edit" and "Delete" should be greyed out, if no address is selected.
b.) If you add a new entry without giving the address (just the label) it says: "The address is already in the address book.". I think it should not even allow an entry without a valid address.
9.) Send Coins dialog:
a.) Having a separate entry field for the [insert missing word] and [again] is a very good idea. It might save some trouble. I also noticed, that pressing "." or "," will send you to the second box. Excellent!
b.) I'd like to suggest a different behavior for the "Add to address book"-shortcut. Remove the checkbox and the text "Add to address book". Simply add the text "Label:" right below "Pay To". The text-field for the address book entry stays disabled until the user inputs (or pastes, or chooses from address book) some address above. Now, if the address is found in the address book, just display the label from the address book. The user has the option to not touch it, or to correct the label which is automatically saved when pressing Send. If it wasn't found it shows a placeholder text with something like "Optional: Enter a label for this address to add it to your address book." If the user enters something it gets in the address book, if not not.
10.) You are giving instructions in your readme how to compile it in ubuntu. On a fresh install of Ubuntu 10.04 I needed the following not mentioned packages to get it building: build-essential, libboost-dev, [libboost-system-dev libboost-filesystem-dev libboost-program-options-dev libboost-thread-dev | libboost-all-dev], libssl-dev and libdb4.8++-dev. Actually it didn't build, because setPlaceholderText in QLineEdit was introduced in Qt4.7 and Ubuntu 10.04 comes with Qt4.6. I commented them out.
Well, that was more to write than I expected. If you made it that far, I'd like to thank you for your attention.

If anything is unclear, please ask. I know I understand my own "english", others might not?
[1] But please translate it to proper english first
