I'v been using Launch Tool for some time now, can be found here with source: http://www.teamhellspawn.com/exl/index. ... launchtool
It's really easy to use and I see qZDL having the same potential. I think the main thing that makes LT work so well for me is the ui layout and engine drop down selection etc.
I think if you were to setup qZDL with just about the same sort of ui system, it would work much easier for others.
If I remember right, Launch Tool only had a hand full of source engines that it would support "out of the box", but several engine configurations can be added by savy end users if wanted, as I did; So it's easy to keep the thing up to date, I actually have it fixed up supporting 16 source engine .cfg's at the moment.
On the side of Doom Connector, well it's for multi-player games of course so being able to chat with other players is a must, as well as getwad support. Without these elements Doom Connector would just be a launcher. unfortunately it can't be used off-line unlike a standard launcher, which is actually imo its down side heh.
But I too think it's one of the best launchers of its class.