Personally, "being as small as uTorrent used to be", but clearly that's not a deal-breaker. (Then again, neither is like two thirds of the stuff GP mentioned!)
To be fair, circa 2010 all torrent clients got "good enough", so my wishlist above is just a wishlist (because torrents are cool and I like to see progress). I use (and sponsor) qBittorrent myself and have no plans to change.