WHAT HAPPENED TO THE WARP SPEED LIMIT ESTABLISHED IN "FORCE OF NATURE" (TNG)?
WHY DO VOYAGER'S NACELLES FLIP UP? WHY DON'T THE ENTERPRISE-E'S?
The Canon Answer:
TNG's "Force of Nature" established that the warp engines of the time damaged the space-time continuum, and a speed limit of Warp 5 or so was introduced to curb the "pollution." Strangely, though, since Voyager's premiere, little mention has been made of the speed limit, leading many to believe that this was just another Trek inconsistency, the writers conveniently ignoring established canon. However, that isn't quite true.
Early in the series' run, Voyager was touted as having "environmentally-friendly" warp engines that didn't have to conform to the speed limit (someone give me a reference here--I believe Ensign Stadi mentioned it during the first "flyby" of Voyager in "Caretaker"); it's been implied (I'm not sure if it was established) that the Variable Geometry configuration (read: "flipping up") had something to do with the new engines' "greenness." We'll assume that this is true, in order to address the additional questions on this page.
The Non-Canon Answer:
The lack of a mention of a speed limit may be attributed to the brilliant scientists in Starfleet hitting upon a solution to cut down on the warp pollution that was simpler that the Intrepid-class' variable geometry nacelles; something like a "warp catalytic converter." New ship designs like the Sovereign-class were probably built with this feature, and older class ships were retrofitted to conform to the new standard.
IF IT'S NECESSARY TO FOLD THE NACELLES UP TO GO TO WARP, WHY FLIP THEM DOWN IN THE FIRST PLACE?
A simple answer might be that Voyager's impulse engines (they're the bumps with red glowing doohickies on Voyager's nacelle struts/wings, midway between the pivoting point and the nacelles) need to be in the "flipped down" position to propel the ship at sublight speeds, but it seems silly that it would be the only reason why--why not just find a better spot to put the darn things?
A plausible explanation is that the Variable Geometry Nacelles don't just have two settings (up and down)...like the flaps and ailerons on an airplane, perhaps variations in the angle of the nacelles play a role in the efficiency of the warp engines. The difference may be a matter of a few degrees (not noticeable to us viewers), but perhaps certain angles are better/faster/cleaner for certain warp speeds or subspace conditions.
Voyager’s "Equinox II" treated us to a shot of Voyager at warp with the nacelles flipped down. Probably a production gaffe, but maybe we’ll chalk it up to our Variable Geometry theory. :D
The Lowdown:
The Warp Speed Limit seemed to be an attempt to cut down on the "Superman factor" in Trek, a problem that plagues it to this day. That is, with all the technological marvels found in the Trek universe, writers have to come up with really cheesy ways to circumvent the tech so that our heroes find themselves in a tough spot ("The supercalifragilistic radiation in the atmosphere is interfering with the transporters"). But since "getting there on time" has never really been a noticeable "Superman factor" problem on Trek--with vast distances between stars and a somewhat clear grasp of what distance can be covered in warp, there is already a sense that there are limits to how fast our heroes can get there, and there was never really a need to limit it further. The writers seemed to realize that the speed limit was an unneccessary obstacle, and wrote it out of Trek lore.
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