The basic constraint is DRC, LVS, Timing, Crosstalk issues.
It depends on what routing you are doing. If it is a power routing, then there are few other things like EM/IR issues that has to be taken care of. If it is clock routing, then double spacing and shielding for high frequency clocks for Signa Integrity purposes. Also in order to meet the timing, needs to do onger or shorter routes.