To amplify on Gray Ham's distinction, new-school hip hop--at least the stuff on the radio, I don't know what's going on in the "underground"--tends to be a bit more up-tempo, often has some classic trance-style supersaw synths (and chord progressions), and is more likely to have "electronic"-sounding drums. (For that last, I mean either drums that were taken straight from an 808/909 or that otherwise have a really unnatural sound to them.) For example,
Nicki Minaj - Super Bass is on the fast side--126 BPM I think--, has a classic "drum machine" bass drum and clap sound, and has a syncopated descending synth that wouldn't be out of place in a trance track from 10 years ago. (I don't mean the main "clinky" sounding synth, I mean the one that's kinda in the background. You can hear it coming in at
2:36 in the bridge; also note that the bridge
starts off with a stream of 16th notes of a synth, with a short decay and modulated/automated filter... again, somewhat trance-esque.)
Here's another example:
Kanye West, Kid Cudi, and Rihanna- All of the Lights is faster and has a pretty pronounced "electronic sound", when it isn't emphasizing the brass parts. (Though it doesn't really have any "trance" elements.)
On the other hand, we still see things like this:
Eminem - Not Afraid. Doesn't really have any of the elements I've mentioned before (other than a trance-like chord progression), but it's not really old school. *Shrug*
Actually, there are probably several different new "styles" that are showing up in contemporary mainstream hip hop. I think the most consistent element is the more obvious reliance on synthesizers and drum machines. (Of course, we saw a ton of drum machine usage in the 80s and early 90s too, though. I dunno.)