I just came across LightWire, anything that stops people sticking createObject() all over the shop helps maintainability, but I always though ColdSpring was light enough - certainly compared to the amount of time Reactor takes to start up the first time.
I don't see a 'killer' feature, and if you are going to do DI to stop your code degenerating into a mess of Italian food, why not go with a more capable framework to start with ?
Of course, having just the one file could be handy, and it looks like it might be more able to cope with people who can't have mappings set up. Having the XML configuration file use the same syntax as ColdSpring also means that you can 'trade up' at a future point, if you watch how you use it.
One to keep an eye on.