I just wanted to add an idea that popped into my mind.
The longer you use Bitcoin, the more transactions you have (d'uh).
Most will not use only a single address, so over time you'll pile up heaps of addresses. Tagging them sure helps organizing, but adding a folder-like system would improve it even more.
I hope the following list illustrates what I mean:
[+] Your addresses
[-] Friends
[-] Bob
|- 2hgZ3hdhdh2... 0.01
|- Hudh38dheUw... 0.02
[+] Jane 1.20
[+] Peter 0.74
[+] Business 12.10
[+] Freelance work 8.41
[-] Various
[+] My website 0.02
[-] Shopping
[+] Newegg -1.33
[-] eBay
|- Bought 1 -1.12
|- Bought 2 -0.08
|- Bought 3 -1.55
|- Sold 1 12.98
That way you could sort in your addresses and group them together easily. Basically like on your computer: you don't store everything in a single folder, but use them to organize your files
How about instead of a folder structure, or in addition to one, the client uses labels. With labels, you can better accommodate contacts who use individual addresses for more than one purpose. What do folks think about the client using Outlook or Thunderbird to manage contacts instead of managing them itself?
Separately, I'd like to see the client have the same functionality as the Block Explorer. Although, that might go beyond the scope of this effort.