I agree on the above definitions of the Impulse Drive (ie gravitic distortion generators), but there's still the nasty problem of directing the heat/waste/crap out of them straight into the nacelle. I'd suggest swapping the P/S drive "pods" on the saucer to extra shuttlebays. At this point, we see the design begin to lean towards the separate bays on the Ent-C and Ent-D. I'd probably give the original central engines more of an orange hue,
ala the TMP 1701.
Apart from that:
I'd eliminate the dual Impulse Deflector Crystals (or whatever) and return the original single unit from the Excelsior. For the colour scheme I'd get rid of the teal bits and re-introduce the blue-grey accents. Those daft neon-blue window lights can go aswell - especially on the saucer sensor dome. Time for some proper white bulbs, thanks!
Secondary hull:
I'd prefer to ditch the outboard flange things - they just looked ugly, IMO. I'd also (as stated by others here) enclose that useless empty space in the ventral undercut and devote it to something more functional or aesthetic - most likely cargo bays. I'd also restore the design of the
aft Secondary hull from the angular blocky structure on regular production models, to the more attractive and curvy (windowed, arboretum?) feature shown on the Prototype Excelsior before she was commissioned.
Torpedoes:
Not to enter fanboy territory, but I'd probably swap out the odd turret things and replace them with stacked rectangular launchers, two port, and two starboard. I expect that Starfleet would find their ships in need of more rapid-fire capability as the various other powers advanced, and the existing technology available not being sufficient to develop this with single (dual) launchers.
Interhull Connector:
This
does need slimming I think, but not a great lot. Maybe take a couple of metres from the total diameter front-back and side-side. I'd also have it as a solid hull piece, with perhaps the forward three quarters aving an inset grill feature - again, leaning towards the Ent-C.
Nacelles:
I'd keep the ones from the Generations appearance, but I'd nix the silly-looking fins from the Bussards, and at the same time remove the ones from the aft ends also. Once again, starting to drift towards the Ent-C a little... The Bussards I don't mind being blue - but then I've
always preferred either that or black versus the red end caps (the whole red/white/blue motif of TNG Federation ships being a bit much for my tastes

)
Internals:
I quite liked the Ent-B bridge... But, I'd get rid of the weird metallic copper/brown paint on the bulkheads and restore the gunmetal grey of the Excelsior and Ent-A. No complaints, otherwise.