First: I am using DDNet-Client and i like DDNet, it is my most played gametype.
I think, that everyone misses the most important point about Teeworlds and DDNet on Steam.
Everyone talks about the clients and the features provided by them, Pathos even said that it is just a matter of semantics and naming. This is not the case. Teeworlds tries to be as friendly as possible for beginners. Having "standard map" and "standard gametype" checked in the filter is one part of it. DDNet tries to be as open as possible but also promotes its own gametype. Let me explain, what i think, is the real problem here:
Nearly everyone of us started Teeworlds not knowing how the game works and as you can see in the Steam forums and on the recently highly populated vanilla servers, the new players are hitting the same wall of first confusion. The standard gametypes of Teeworlds are easy, DM and TDM can be understood by just joining the server and seeing the points go up by just hitting other players, even though you have never played a game even remotely like this. CTF needs a bit more time, but can be understood nearly as fast if played with others that get the concept. Why am I talking about gametypes, while everyone else is talking about client features? Because the DDNet client promotes the DDNet mod - that is vastly different from the "only vanilla approach" of the Teeworlds client. I remember that my first contact with DDRace and DDNet was a different experience than the contact with DM, TDM and CTF. Don't get me wrong, as i said, DDNet is the mod I play the most, but at the beginning I hated it. The servers tagged with "NOVICE" are much harder than any DDRace beginner server in the past - this is what i think is the pitfall. People already started to complain about Teeworlds in the Steam Reviews, because "there are too many good players, I can't even hit anyone and die constantly", with DDNet hitting Steam, this is not going to get better. Vanilla Teeworlds explains itself, DDNet is not. On this basis one could also say: "But Teeworlds also does not explain things like Demo-Recording and the map editor" and you are totally right. I think it would have been better to wait for 0.7 and release it on Steam, because this version actually features a main menu listing the map editor and better user interface for demos.
The next thing is about the "standard map" filter. The official Teeworlds maps have consistency. Everything that looks like it is an unhookable, is an unhookable. DDNet maps often have to explain themselves in the starting room. If you have to explain the most basic parts of your gameplay for every map, there might be something wrong (I know that this is an historical thing, born from the fact that Teeworlds had no tileset with 1x1 tiles apart from the unhookables, but it is nevertheless confusing for first time players).
I don't think that DDNet should not be on Steam, it has the right to be and many good standalone games started on Steam as a Half-Life mod. But in order to be widely accepted by the new players, it has to explain itself better. Give the new players a playground to test the mod and help them. And no, the NOVICE servers are not a good start here.
I hope that this is considered by deen and the DDNet staff, as a bad experience with DDNet could ultimately draw players away from everything Teeworlds related. At the moment the new players join DM, TDM and CTF - understand it - play it - think it is good and stay there for some time. Or they think there is something to be desired, but then hey are able to change the filter and experience mods, made by people that had the same feeling. In the future, if nothing is changed for beginners, they may download DDNet, join a DDNet server, be overwhelmed and leave (fact: this is what i did the first time - and i did not go anywhere near a DDNet server for many months after this). These players did not experience Teeworlds core gameplay first, they have no idea why they should stay and give it another chance. And because DDNet looks like Teeworlds, it may hurt Teeworlds, too.
Conclusion:
Teeworlds on Steam needs to hit 0.7 as fast as possible, removing the last questions every new player is asking over and over.
DDNet on Steam has to be at least as friendly for beginners as Teeworlds 0.6 vanilla - otherwise I see an awesome mod ultimately killing the game as a whole, just because it wasn't inclusive enough.