The main way you can increase your review scores is by having higher overall values for Fun, Graphics, Creativity and Sound. These values, in turn, are mostly affected by your staff members, so hiring the best staff is extremely important. Development of a game takes a certain set time (controlled by the budget you set for it) so the only way to consistently cram more Fun into the game is to have staff members that generate more stats-per-second. The high-cost, high-stat staff members regularly churn out +8 or +10 without even being "on fire", so the more high-stat workers you have, the more Fun/Graphics/whatever your game will have. On top of this, the high-stat workers will contribute MUCH more when given one of the three main jobs (Scenario/Graphics/Audio) for a game. I've had the Graphics phase end with the guy adding +103 to the Graphics category before!
One example of the stats you need for high reviews: In my most recent playthrough I released multiple games with two or three of Fun, Creativity, Graphics, and Audio over 200 (and the others not far behind) and I still never got a perfect 40 on the reviews. You definitely need to start pushing 100 in each category to get really positive reviews, I would say.
Some of the game text would seem to suggest that good genre/type combinations only affect sales, but I have seen review comments about the weird combination, so I think particularly bad combinations may negatively affect review scores. It's better all-around if you can find a good combination, of course, since you'll earn more money that way.
In order to get the best staff members (with 200-300+ ratings in Coding/Scenario/Graphics/Audio) you're eventually going to have to sink money into the costly Hollywood Agent recruiting type - you just won't get the best people without spending that $3.5M. You'll also need to have plenty of excess cash to pay the hiring contract cost for the best employees (which can be several million as well).
Getting just one Hacker or high-level Director or Producer into your company will really help, boosting the scores on your games immensely and letting you hire even more talented people. These high-stat workers can even usually do two or three roles per game, writing the scenario, doing the graphics, and even maybe the audio (as long as you rest them once in a while).
Leveling up and training your existing employees will help them generate more Fun etc for the games, but ultimately not everybody is equal - you're going to have to fire some of your original employees to hire the big shots with the super stats (like Francoise Bloom, Walt Sidney, and Stephen Jobson).