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.
I've pitched CMake in the mailing list. I wonder what the reply will be \o/ Some people might be annoyed though because they went through all the trouble to port it to autotools which is no small feat. On the other hand, Mac/Windows was problematic with autotools.
The discussion seems to have died out. I've bumped the thread.
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.
The "hidden address" thing using a flag is a good idea, both for receiving and sending addresse (for the reasons that you mention). We'd have to be really sure that this is safe, though. I'll have to think about it a bit.
Currently, it's possible for (receiving) addresses to not be in the address book at all. Those are considered change addresses by the wallet interface. I don't think that's exactly what you need, since deleting an address on which you've received money would confuse the user interface.
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 I think the "Set as default receiving address" is obsolete now?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).
I think so. I don't really understand the default receiving address stuff either. I don't think it's used anywhere in the Bitcoin core code. I need to check this and if that's the case, completely nuke it.
The current proposed wallet encryption patch will use the default address for generations, if no fresh pool key is available. I doubt it's necessary to show this in the user interface, though. It seems like a legacy way of showing people "this is the address you should use".
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.
It should indeed check addresses for validity before adding to address book.
See pull request 358.