Besides have generally good and original writing, the thing I find most compelling about this series is that they aren't afraid to address the grey areas of the law. So many crime dramas try and paint the world black and white: the "good cops" versus the "bad In SVU, sometimes the cops use disgraceful methods to achieve their goals; sometimes there are consequences, sometimes not. We also see suspect who appear abhorrent, yet technically not guilty, or released because of some problem with the trial or investigation. There are episodes where it's not really clear who's the victim and who's the culprit. Finally, sometimes it turns out there is no crime at all!
This ability to show the "grey zone" in fighting sexual crimes is a much more interesting reflection of reality than what you might expect of a rape crime drama.
In earlier seasons the show often ended inconclusively, whereas in the last few seasons they almost always follow through to show the result of the trial. I enjoy both approaches, though I suspect the former takes more guts to write.