Nuendo is a much more powerfull tool for audio. it's a lot more stable, ability to playback more tracks, etc.
the audio editing facilities is much more aimed at professionals in Nuendo. it's also little things like as you're recording, it shows you the waveform in real time, like in Pro Tools. it's quite similar to protools in a few ways.
automation editing is a lot better, surround sound mixing capability, 32 bit floating point mastering, lots of applications for nuendo.
it has a much more solid feel than cubase. i still run Cubase, and I run either Pro Tools and sometimes Nuendo along side Cubase. i dont care for the midi much in Nuendo. it's still compatible with VST plugins (such as Waves)
It has the capability of importing cubase files into Nuendo which is great. it can import EDL's from Premiere, has support for remotes, you have to try it.
the thing that attracted me was that, i get a lot of people bringing in stuff done in Cubase for me to mix, so Nuendo was great because it has a solider, professional feel to it, and i could load their cubase stuff into it.
-Romesh