The wifi reception of the old device was the worst thing about it. There were occasions where my phone and tablet had absolutely no trouble seeing a wifi network and the chromecast was completely unable to. So if they've improved that then that's good I guess.
The thing that would make me happiest is if chromecast were able to work around dumb public / hotel hotspots that were protected by confirmation / registration screens. Even if that means shipping a stripped down browser that could let me sign onto the network. Getting it to work at the moment is a dark art and usually requires running through another device as a hotspot / bridge.
It'd be nice too if it natively supported MKV too. I don't see why this is a big deal to withhold - it's just a container that has to be demuxed - the audio / video / subs would still have to be the correct format.