We've been living with Rubicon for about a week now and it's a difficult expansion to describe. The best I can do is on the surface it's very light on "expansion" but if you drill down a little deeper it does have some gems.
The warp changes, as outlined here, are pretty awesome. Sit at a gate and watch the various ships warp in. Instantly you can pick the mass differences as they leave warp. Freighters come in slow and eventually come to a stop. Interceptors seem like they're still in warp and then just stop right at the gate, like a hyperactive kitten that's just seen something it doesn't like, and then they're off again, zooming away! In the day after Rubicon was released, once the launcher issues were overcome, the local system entirely consisted of twelve guys racing around in their interceptors laughing their heads off.
It was quite amusing.
The warp changes also help game immersion as now the mass of a ship matters while entering and leaving warp, it makes the Eve Online universe more visually believable.
Another addition that came along with Rubicon is the login character screen. Not something that adds to game play but this feature is top of the list when I've asked others about Rubicon features they like.
Marauders seem pretty cool but some fittings evolution will happen over the coming weeks and months until we reach some seriously tough ships.
Finally the changing of the confusing certificates system to the mastery system seems to be working very well. Essentially you just find the module or ship you want to improve, view the mastery tab, see what has to be trained to move in rank from level I to level V and that is it. Simple!
The rest of Rubicon has not grabbed my attention very much at this stage. The only new deployable module I can see being useful is the portable cyno jammer. The Sisters Of Eve (SOE) ships are very over powered and will be nerfed in the future, thankfully the typical CCP haste will be applied so those pilots have a fair bit of time yet.
I look forward to the yet to be described Rubicon 1.1 and now the Summer (my Winter) expansion a very long eight months away.