There are actually aren't that many good ideas in there. 99% of the premise is identical to the Mondaiji light novel series which was published a couple of years before the guy started NGNL: OP heroes are summoned to a magical world where all wars are resolved by playing games. They are summoned by a faction which is at death's door, so that they can win at the games and reverse the losing factions chances. After winning each game, they gain lands and resources which are used to improve the life quality of the residents. In the end they must face ever more powerful enemy races until fighting a final super-boss. That's Mondaiji summed up, and it's also exactly what NGNL is about.
The only difference is that in Mondaiji the games are like magical gladiator battles, and in NGNL the games are real-world games of any type. In NGNL everything is based on games, so there are 16 races and each race corresponds to a "race piece" which is a chess-piece. So you get magical Chess, magical Othello, magical Rock-Paper-Scissors, etc. So, for each major "battle" he just picks some real-world game, and them concocts some magical "twist" to the rules. That's the only part of the writing which involves any real creative thought, and critics point out that he can't even get that right most of the time.
Some NGNL fans have said the similarities are merely coincidences, as they are themes common to many literature, such as humans summoned to be heroes in a fantasy world. But the NGNL guy was also caught red-handed plagiarizing art from other popular light novel/manga series extensively for his cover illustrations. He was tracing the poses from other works to get his character poses down for the illustrations. Sure, he adds his own little flourishes, but they're still traces of the main structure of the drawing. The Japanese were able to pinpoint the source for cover-art of NGNL volumes 1-6. NGNL fans tried to claim the tracing allegations were "merely coincidences" too, and that you could make the same parallels with anyone's art by looking hard enough. But then Volume 7 came out after the allegations and the Japanese put it through extensive scrutiny and couldn't find any evidence of tracing for that one. Sort of proves the point, doesn't it? If it was "just coincidence" then you could do the same trick for the later volumes as the earlier ones, but you can't do that. The guy is a plagiarist.