The most important feature is that private torrent websites accept it. On private sites you get incredible speeds, dedicated seeders, and either the newest content on some sites or specialized sites with all kinds of rare files. On the forums in www.zeropaid.com there is a thread noting what private sites are open for signup. Oh and uTorrent has encryption and works for private sites.
utorrent is the best of these. forget about azuerus too. transmission looks interesting.
high efficiency and low processing is what will catch on among nerds/competent, so stick with utorrent.
