The Enterprise-D's maximum sublight velocity/acceleration.

[Roondar]

In ST:TMP, the Enterprise's warp drive was still untested, so it makes some sense that they held off on using it until they were well into the outer solar system.
-Mike

A good point, and it may also apply to systems that are unfamiliar to the UFP, and that it's simply a rule of thumb to drop out of warp because of that. Although why they did it in BoBW is a bit strange. I can see the Cube doing it out of a tactical point of not chargining into a well defended system, but the Enterprise D should have kept at warp. Although, Earth just may be one of those systems, or perhaps another reason might be attached to it (such as a paticular time or year, or area might be alright, but otherwise no).

It could be that their angle of approach would mean they'd bump into one or more 'obstacles' (read planets or other objects) if they had kept warping?

For better or worse (i.e. it doesn't make much sense but still appears to be true) the Sol system does appear to have an above average planet-count in the ST universe.

[Mike DiCenso]

Oddly enough, the E-D and the Borg cube came out of warp near Saturn, which as we understand things now is pretty deep inside the solar systemfrom a relative standpoint. There is no reason why either one could not have continued on at warp until very near Earth.

As for E-D impulse engine acceleration, another possible data point can be found in "Descent, Part II", where the heavily damaged E-D escapes from the Borg ship by heading towards the system's star at full impulse. It takes several minutes, which seems to indicate the velocity and speed was a high percentage of c, assuming that the planet the E-D left orbit from was the same approximate distance from it's sun as the Earth is from Sol.
-Mike

[Roondar]

It's of course possible that the Borg's (and E-D's) deflectors could handle the smaller objects in the belt but could not manage the bigger things (like Mars and Jupiter) ;)

Or perhaps the concern of other ships or even defensive measures of sorts caused them to slow down.

As to Descent, II. That seems like a reasonable assumption, assuming that we can make sure it did indeed only take a few minutes. I can't recall how the scenes went so I'm not 100% sure we actually get to witness the travel purely from the E-D's perspective.

[Mike DiCenso]

The E-D's run to the star in "Descent, Part II" takes at least a 2 minutes of screen time. Doctor Beverly Crusher gives the command to take the ship into the star, then there is a cut to commercial break. When the episode resumes it picks up with an FX sequence of the E-D heading in towards the star with the Borg ship in hot pursuit. It then cuts to a conversation between Crusher and two young crew members about how to implement the metaphasic shield program, and finally once the metaphasic shields come up, more FX scenes of the E-D entering the star. About 3 or so minutes.

Most of these scenes are from the E-D's perspective, but there are definite jump cuts. Most especially that first one that indicates maybe a minute or two went by before the metaphasic shield discussion scene picks up.
-Mike