Well, the big obvious one is that it voids your warranty. No manufacturer will offer you support or warranty service if your device is rooted. Telecom companies might, mostly because they don't care and the workers they employ don't know better, but if you want support from the people who made your phone, you'd be out of luck.
Another disadvantage is that you would no longer get OTA updates. By rooting, you exclude your device from the automatic system updates that get pushed out. You can still apply these updates manually, but it requires a few more steps and can be a hassle. This is less of an issue because most people who root their phones use custom ROMs that don't get official OTA updates anyway, but if you're using stock Android...