Whenever someone complains about a reviewer being too "subjective," I' think what they are actually complaining about is that the reviewer has a different opinion than them.
I know a couple of people I talk to use this term when what they actually mean is that the reviewer just doesn't provide a lot of concrete information to justify their opinion or to explain systems and mechanics.
e.g. someone saying something like "this game has some fun combat mechanics, and does a lot of interesting and unique things that make it a must buy", and then not detailing the components that are interesting and unique and what makes it standout/highlighting why they find it fun so I can judge if it's something I find fun.
So mostly just using the wrong word to describe why they take issue with it.
Edit: Oh and I know people were shitting all over EmptySpace in this thread but the one thing he said that I do, personally, agree with to some extent is that reviewers should be reviewing games in genres they are interested more often than not. It'd be nice to get two reviews, though -- one from a genre fan and one from a party with less familiarity with the genre. There have been a few titles I know I've read a lot of bad reviews on that contained information that made me feel maybe the game was a weak genre entry, but once I impulse bought it on the cheap I actually really enjoyed it (as did others I then loaned it to). There's a fine line to tread, of course, because superfans of a series or franchise might hype it up, but it's always good to get the SRPG guy reviewing SRPGs and the sports guy reviewing sports games. I feel a lot of the nuances are better examined under that light.