Valve have brought out changes to how scores for Steam games are calculated by ignoring the impact from "off-topic review bombs" by default. So, what does that actually mean and what are "off-topic review bombs"? Well, Steam's algorithms have a way of flagging "anomalous review activity" (such as a lot of negative reviews within a short period of time) and these are apparently reviewed by the team at Valve. If the team determines that this period of reviews is an "off-topic review bomb" they'll mark it as such and notify the developer.
Valve ensures that none of the reviews will actually be removed but their scores will be ignored by default when calculating the score. They also stated that if a user still wanted to include these "off-topic reviews", you could do so through the review settings.
But what does Valve consider to be an "off-topic review bomb"? Well, they've only made explicit mention to two so far: negative reviews concerning DRM or EULA changes. Unfortunately, anybody who happens to write a "legitimate" negative review during the same period as an "off-topic review bomb" will also have their review ignored when it comes to calculating the game's score.
Of course, you could just ignore all this drama and just read reviews you can trust right here on Choicest Games ;).
Oh, if you ever wanted to keep things the way they were (i.e. include "off-topic reviews") you have to go about it in a convoluted way (as it's not actually a radio button in the user reviews window on the Steam Store page):
1. Click on your profile name in the top-right
2. Select Store Preferences
3. Scroll to Review Score Settings
4. Click Edit Preferences
5. Select Include reviews from all Steam purchases in Review Scores
[ Steam: User Reviews Revisited ]