ok this is what my dad told me that is important in a rice cooker. he works at a tatung , they manufacturer rice cookers and HP's pavilion line of computers and sun workstations and various monitors. I still dont understand how rice cookers go with the rest of their products. Anyawys... the idea is ... he says is the steaming ability. If you have a metal ricecooker, and the lid is designed to expand outward as more steam is generated then it will seal the edge of the lid instead of releasing more steam. This will force the pressure of the steam to go more into the rice and force more water into the rice, thus cooking it better. Also it is good if there is an internal like rice pot inside, that is held inside the metal cylinder. that way you can pour extra water outside the main rice holder metal pot, in the cavity between the heating element and the rice . this extra water will also be steamed into the rice. we had a 20 cup rice cooker at one point that he brought in from work, we never used it though. Jack in the box used to buy their rice cookers for the terriyaki bowls there, so well that was the type jack in the box used.
Anyways, most el cheapo american rice cookers do not make a nice seal on top . The really cheap ones even have a steam release on top, because they can not seal effectively. probably would just bust the lid on top if there was no seal on top. Anyways, the REALLY REALLY cheap ones, have glass lids. glass obviously cannot take the pressure of a lot of steam, it will just crack. A good rice cooker uses a heavy metal usually stainless steel lid, or one that locks so that the top will not lift and will keep the steam in. Basically a rice cooker is a pressure cooker so well a glass pressure cooker really doesnt work. neither does a plastic one, well not as well.