I think that TNG is too narrow of a descriptor. The ships of the 2360's, ie: Galaxy, Nebula, etc... are products of schools of design of the late 2340's and 50's, if you read the tech manuals. So really, TNG I think means 2350's and 60's.
My thoughts:
1: A saucer that is wider than it is long.
2: Very smooth, organic lines. No hard edges like what we see in TMP or the revival post-TNG.
3: Wrap around blue nacelle grills.
4: A galaxyesque color scheme.
5: Impulse engines that look a little small for the design.
6: Phaser arrays, rather than emitters.
7: Nacelles mounted below the saucer, whether or not they are above or below the drive section.
8: Saucer sep
These are mostly things that could be executed in a variety of ways, but i think they certainly help to give a TNG feel. I think that #1,2,3, and 6 are probably the most important.