If NICE/Veolia can actually follow through (as opposed to simply having a service increase "on paper"), the changes generally sound like good news for many Nassau County residents and those who commute/travel there for school, work, etc.
That said, I can't help but notice the way NICE/Veolia is going about some of these changes resemble the behavior exhibited by certain teenage students (which we teachers semi-affectionately called "knuckleheads") in a somewhat dysfunctional high school I used to teach in. We (teachers) usually figured that the "knuckleheads" have it all in them to be competitive, smart and become the pride of the community. But for some awful reason, they just cannot do things right the first time or at least come close to the mark. If they do manage by some miracle, they always seem to find a way to undermine themselves and their work to everybody's frustration.
For what it's worth, my two cents on the proposed changes (I'll limit my thoughts to which I've had a chance to ride extensively in the past):
N4X - I agree with others on this thread who stated that it would probably be prudent to add a few more stops on this new route. As is, it looks to me like that the new route bypasses some "major stop" locations and will adversely impact the local buses trailing the expresses.
Since I have never ridden past Rockville Centre on the N4, the only extra stop I can confidently nominate is Central Avenue in Valley Stream (connects with N1, almost always has passengers waiting to board/alight at the stop all times of the day, near South Nassau Community Hospital and reasonable walking distance to nearby retail stores,).
N6X - I agree with Brett all the way here. Just bring back the damn N6 Limited and call it the "new and improved N6 Express." I promise nobody will roll their eyes on that one. It was one of the few things from the old regime that still worked very well to the end.
I would suggest having more N6X buses starting/terminating at HTC (perhaps as a slight majority) than N6X buses terminating/starting from NCC. Ideally, tweak the service proportion based on academic year and timing based on a fair margin before the majority of morning classes start and after the majority of afternoon/evening classes end.
N21 - Last month, I finally got a chance to go to Glen Cove on public transit. Damn, are visitors/residents there who rely on public transit screwed if they ever have to go there or leave during off-peak hours! (The LIRR won't help much either, with once every other hour service!)
Perhaps off-peak only, the N21 can be re-routed to serve Greenvale and otherwise duplicate the N27 route going into Glen Cove (ideally so that there is an offset between the N21 service and N27 service). Yes, it will add some running time, but I figure it may be justified by picking up more passengers that way than going along the scenic Sea Cliff, Glenwood Landing and Roslyn Harbor route.