1. Download this file.
2. Try to open it in kTorrent.
3. Try to open it in uTorrent (works fine with Wine).

Actual result: uTorrent can open this torrent but kTorrent can't and show error message.

It is not a valid torrent.

A torrent needs to have an announce field (with the tracker URL) or a nodes field (a bunch of DHT nodes to contact), this one has neither of these.

This torrent is VALID. Why? Because I downloaded it in uTorrent without any problems. So, DHT-nodes AND tracker isn't necessary at all.

I tied to find information about torrent file format and find nothing except this:

* "announce": (optional) The announce URL of the tracker (string)
* "announce-list": (optional) multitracker list (a list of string)
* "nodes": (optional) A list of DHT Network nodes, each nodes is encoded as a 2-element list of host(string) and port(integer)
* "private": (optional) if the torrent is PrivateTorrent(SecureTorrent), that is to announce only in tracker or tracker-list (integer, 0 or 1)

All these sections are optional. DHT is based on autodiscovering of nodes. If there is no predefined nodes then kTorrent will find them automaticaly.

Also I tested this file in Deluge. Deluge can open and download this torrent!

DHT spec (http://bittorrent.org/beps/bep_0005.html):

Torrent File Extensions

A trackerless torrent dictionary does not have an "announce" key. Instead, a trackerless torrent has a "nodes" key. This key should be set to the K closest nodes in the torrent generating client's routing table. Alternatively, the key could be set to a known good node such as one operated by the person generating the torrent. Please do not automatically add "router.bittorrent.com" to torrent files or automatically add this node to clients routing tables.

nodes = [["<host>", <port>], ["<host>", <port>], ...]
nodes = [["127.0.0.1", 6881], ["your.router.node", 4804]]

Nowhere does it say that the nodes key is optional. Further more if your routing table is empty, no nodes key, means nobody to contact.

I can get rid of the check for this, but in my opinion it is still an invalid torrent.

Probably this torrent is invalid... but how I downloaded it if it's invalid? uTorrent and Deluge can download this torrent, why kTorrent shouldn't? I propose to replace blocking by warning. kTorrent should show warning about not proper state of torrent and try do download it. If it can't -- at least user was warned. If it can -- great, user have what he want.

I've tested this torrent more:
I've kept this torrent in Deluge for seeding and now I see how people downloads it from me. So, I can download and seed this torrent without any problems. Predefined DHT nodes is not necessary for downloading/seeding DHT-only torrents like this one.

So, this torrent is completely valid.

The check has been removed, though in my opinion it is still an invalid torrent.

Thank you.
Actually I can understand your position. Seeding and downloading in such cases works really slowly... but it works. Someone made torrent-file without trackers and DHT-nodes but who will be victim of this mess? End-user. He will be blocked from downloading when he can download something. I think it's totally wrong.
Also this coin have another side. What user without experience in kTorrent will think when he tried to open such torrent by kTorrent and Deluge (or uTorrent, or Vuze, or whatever else)? He will think: "Looks like kTorrent have problems even with opening of torrent-files, probably this program bugged like hell!" Of course he will chose another program and will use it in future. kTorrent became best torrent-client for me and I don't want to use uTorrent or whatever else but he will chose something other because of some unknown guy and this validation check. This check just isn't user-friendly and nothing more. That's why I asked you to remove this check.