What are the differences between the 'base' of various Linux distributions?

I've been using linux desktop for a year or so now. One noteable thing i keep seeing is that one person will say I dont like XYZ distrobution because of its base. But I am still a little unsure what is meant by it. I am assuming the main difference between each base is the choice of package management(?). But what other factors/aspects that are important for the average user to know about each 'base'? This is probably quite a broad question to a rather technical answer, but appriciate any answers, and i'll try my best to understand and read up :)