If our planet was without its ocean but otherwise the same as it is today, would surface temperatures be more extreme than they are now (that is, higher high temperatures in summer and lower low temperatures in winter), of less extreme, or what?

They would actually be so extremely hot that the whole earth would become a desert without any real seasons. The lack of water would also make it unable to sustain human life. Then slowly the planet would most likely have it's atmosphere in short exclamation, "It would go away." And then our planet would probably end up looking very similar to the red planet -- Mars.