This is a list of most of the already requested features.
Please refer to it before posting a new suggestion.
(I'm adding links to some of them, be patient.)

General -
¤ Skin Support (1)
¤ Password protected adult filter. (1)
¤ Show accurate horizon statistics (for leaves too)
¤ Integrated video player
¤ Allow preferences to be imported/exported
¤ Capture crash information, for reporting
¤ Make Client Multi-Language
¤ Dynamic domain name support. (1)

Connection -
¤ If a user selects "Modem" as his/her choice of connection, then automatically set "ultrapeer capabilities" to disabled.
¤ Autodetect and force IP address.
¤ Anonymity Options
- Add selection to Hide last one or two octets of your address to users who request it. (add to next version of gnutella protocol?)

Chatting -
¤ Place a name in your client, for chatting.
¤ Away message for Chat reply
¤ Timestamp for chat window. (1)
¤ Chat alert sound/chime. (1)

Network Health -
¤ Limit swarm downloading for Modem(56k) users to 2 slots per download.
¤ Encrypted File Transfers between users(via Blowfish/Twofish & simple Key Exchange, similar to SecureIM in Trillian.).

Content/Searching -
¤ Implement Less than/Equal to/Greater than searches (Music -> Greater Than -> 160, all music 160 and more will be searched).
¤ Re-implement the "browse host" feature
¤ Add remote download queueing
¤ Dynamic result sorting
¤ Hashing
¤ Eliminate "Incoming Searches" window.
¤ Bogus File Filter - (Unshare, and dont search for music or movies under 20kb in size) Do file integrety checks. Verify MP3 files as actually being music, etc... (1)
¤ Related File Searching (1)
¤ AV Signing of Shared Files (1)

Downloads -
¤ Estimate Time Remaining for Downloads.
¤ More Agressive Swarm Downloading. (1, 2, 3)
¤ More Agressive Download resuming.
¤ "About User" (get):
- Users current # of Downloads
- Users current # of Uploads
- Connection Speed
- % Bandwidth Upload Amount
- MB Shared, # Files Shared
- IP Address (Hiding Last Octet.)
¤ (Pause/Resume/Cancel/Find More Sources) options in right click, for Downloads.
¤ An indication that your download attempt was block by the "Anti Freeloader" option.
¤ Add option for Virus scanning of files (go!zilla style, query Norton/McAfee/Whatever to scan just the one file.) (1)

Uploads -
¤ Estimate Time Remaining for Uploads
¤ Special "Express" Upload slots for files 500k or less. (1)
¤ Turn off "upload limit reached" messages (1)

Anti-Freeloading -
¤ Implement a rating system which allows the exchange of host ratings peer-to-peer; for example, if host 'A' shares a file with host 'B', host 'B' could broadcast a positive rating across the network which includes the file size, download time, etc.; ratings would be kept on individual hosts and shared via the network.
¤ Add the following bandwith types: 56k, Dual ISDN, Cable/DSL, T1, T3.
¤ Ask the user during install if he/she wishes to "share files with people who dont" and if he/she says no, then set freeloader filter to "rarely" and minimum files as 10.
¤ If a user is 56k, dont mark them as a freeloader, and have upload and download caps that would keep a cable user from saying that he/she is 56k.
¤ If a user selects "Modem" as his/her choice of connection, then automatically set an unchangable total -download- bandwith cap of 10Kb/second.
¤ If a user selects "Dual ISDN" as his/her choice of connection, then automatically set an unchangable total -download- bandwith cap of 20Kb/second.
¤ If a user sets Start Upload slots to 0, Max Upload slots to 0, or Max Uploads per person to 0, then mark that person as a freeloader.
¤ Have an unchangable minimum of 1 "express" upload slot (for any bandwith)(see above, uploads).
¤ Display pop-up notices encouraging users to share if they're not.