It may seem like you can test everything in Tetris just by playing it for a few minutes, but this is very unlikely! As I explain in this article, the game is filled with special cases that rarely occur in normal play, and these can only be easily found with the help of a coverage tool.
From eyeballing relative performance expectations, the GTX 970M should be well-suited to 1080p or below at high detail levels, while the GTX 980M should be capable of ultra detail at 1080p or higher resolutions. Maxwell's better efficiency means that it should offer a significant performance improvement over mobile Kepler, even with the same number of cores. Also with this launch Nvidia is introducing "Battery Boost" as a solution for games with less demanding graphics, where battery life can be extended by governing clock speeds to maintain playable frames, without overpower the GPU at higher than needed frame rates.
The current "user rating" ranking system seems to divide everything into big semantic buckets ("Overwhelmingly Positive", "Positive", "Mixed", etc.), stack those in order, then sort each bucket's contents by the total number of reviews per game. Given that Steam reviews skew massively positive, (about half are "very positive" or higher), this is virtually indistinguishable from a standard "most popular" chart.
Luckily, there's a known solution to this problem — use statistical sampling to account for disparate numbers of user reviews, which gives "hidden gems" with statistically significant high positive ratings, but less popularity, a fighting chance against games that are already dominating the charts.