I'm new to S3D and haven't been able to find a workaround for this yet, so adding it here as a feature.
My printer (Sailfish based firmware) handles acceleration, which mostly works. However, it seems to base the acceleration on the speed of the extruder, not the velocity. Therefore, if you have an abrupt direction change (making something square, for example) although the speed may be constant as the extruder traverses the X then Y axes, there is a big change in velocity as the direction changes which results in vibration and reduced print quality. I assume many other printers are the same.
For optimal printing it is essential to reduce the velocity at sharp changes in direction.
I previously used Makerbot Desktop which did have a feature to avoid this called "doDynamicSpeed". This ensures the extruder slows before a change in direction, then accelerates again back to normal speed. Comparing test prints from Makerbot Desktop and S3D this is the only place where S3D falls down (so far, all other aspects of the print are better with S3D).
There are at least two ways this could be added:
(1) Correctly modulate the speed around sharp direction changes (something like this is already done for "small" features; it just needs to be enhanced to support sharp direction changes).
(2) Modify the Gcode so that paths are split into separate paths. For example, an "L" with a sharp corner could be changed to be two linear paths with a pause where they join and allow the printer firmware to handle the acceleration on the paths. This would probably only work for straight edges, though.
Note, there is a similar feature request posted (viewtopic.php?f=23&t=9375) but it isn't quite the same thing.