Ability To Keep Coffee Warm

Neither porcelain nor ceramic is particularly good at keeping things warm. They don't transfer heat particularly well, though, which is why they're typically used as coffee cups. The whole point isn't to keep the heat in so much as to give you a handle that's cool enough for human hands. Glass typically transfers heat a bit better, but glass coffee cups are generally made to be at least as heat resistant as more traditional ones. Of course, nothing beats an aluminum or plastic insulated mug, and even disposable coffee cups with lids will keep coffee warm longer.