Are there any negative effects to kneading bread dough longer?
There are several negative effects from over-kneading bread dough:


Overheating - if the dough gets too warm, it will ferment too quickly (or over ferment) and will therefore lack flavour.
Oxidisation - kneading for too long can cause the flour to oxidise and bleach, again impairing flavour.
Breaking down - eventually the molecular bonds of the gluten will break, which is obviously not what you want to happen!


The latter two are really only possible with electric mixers, however.

Most doughs are ready for fermentation when they reach an internal temperature of 77-81ºF (25-27°C). You can also check the gluten development with the Windowpane Test: pull off a chunk of dough and stretch it with your hands. It should stretch to form a very thin translucent sheet, without tearing.