I haven't checked the reviews, so I don't know of any actual problems with it, but in a lot of ways they set themselves up for failure.

It's been in development for what feels like fucking forever, so expectations are high. It's the successor to a massively popular game that influenced so many memes and a decent bit of online culture, so even without bloated expectations it has huge shoes to fill. Lastly, it's the sequel to a game that relies entirely on user made content to thrive, so there was always going to be a big gap (which may never actually be filled) while you wait for users to make new content for the new engine or to transition existing content over.