Final:
Romania and Latvia met twice before at the World Cup (at the tenth, and the twenty-fourth) - they each won one of the matches (Latvia won the last one). The two also played in the same qualification group twice before (for the eighth, and the thirteenth World Cup) - Latvia won two, Romania won one, and one ended in a draw.
Latvia had a good semifinal match. While they and Serbia were fairly equal in most areas of play. Latvia created more chances for themselves, and they were the only ones to score in the end. Romania and South Africa had an exciting semifinal match. South Africa were a bit unfortunate to lose, especially the way they did.
Romania are the tentative favorites. Their team has well rounded players, capable of taking more than just one role. However, good tactics could easily overcome the minor skill differences - underestimating Latvia would be a costly mistake.
Romania vs. Latvia

3^rd Place Play-off:
Serbia and South Africa played three times before at the World Cup, including in the group stage, here. Before this World Cup, Serbia won both matches (at the twenty-first, and the twenty-fifth World Cup), but here, in the group stage, South Africa won. The two nations also played in the same qualification groups twice thus far (for the eighth and the eighteenth World Cup) - Serbia won three of those, South Africa won one.
Like in the match for the World Champions, here too we have a team that is technically better - Serbia have a well rounded team, and are considered the tentative favorites. However, one just has to look at South Africa's opponents to see that they already defeated quite a few "technically better" teams, including Serbia in the very first group stage round.
Serbia vs. South Africa

Final:
Somewhat surprisingly, Netherlands and Poland played only three official matches thus far. One was at the fourth U21 World Cup, and two were in the same qualification group for the tenth U21 World Cup. Poland won the U21 World Cup match, and Netherlands won the two qualification matches.
Even though it took them a while to score, Netherlands had an easy semifinal win. Croatia's Nikša Pavlovic received a red card in the first half of the game, and it was only a matter of time when Netherlands would score. Poland won their semifinal game with 2-1. However, Moldova created quite a few chances, they just struggled with the finishing.
Poland have a slightly better team than Netherlands, and are the tentative favorites.
Netherlands vs. Poland

3^rd Place Play-off:
This is the first time Moldova and Croatia meet at the U21 World Cup. They have however, played in the same qualification groups four times thus far (for the fourth, ninth, fifteenth, and the twenty-third U21 World Cup) - Croatia won six of those, Moldova won one (their last home match), and one ended in a draw.
Croatians are the tentative favorites here. While they lost one of their best central midfielder to a red card, they have adequate replacement for him (one that is still stronger than his Moldovan counterparts). However, as in other games where we have a favorite, underestimating Moldova would be a mistake.
Moldova vs. Croatia

Final:
Argentina and Poland met only twice before, and both games were at the U19 World Cup (at the fourth and the eighteenth U19 World Cup) - they each won one of the games.
While the game between Brazil and Poland was fairly even, possession wise, there was a huge difference in how the two teams created chances. Poland stopped almost all of the Brazilian attacks before they could be truly dangerous. And the couple Brazil managed to pull together were stopped by Czajkowski. Argentina keep surprising us at this U19 World Cup. They showed no fear when facing off against Romania, and had an excellent 0-2 victory.
While Poland have a slightly better team than Argentina, and are the favorites today, discounting Argentina would be a mistake. Their manager has proven that the correct formation and good tactics can overcome the minor differences.
Argentina vs. Poland

3^rd Place Play-off:
Romania and Brazil met twice before at the U19 World Cup (at the fourth and fifth U19 World Cup). The two also played in the same qualification groups twice thus far (for the second and eighth U19 World Cup). All of their matches are so long ago, that the majority of players playing then, already retired. But, for the sake of statistics, Romania won all but one of the matches (the one qualification game ended in a draw).
Brazil have a slightly stronger team than Romania, and are the tentative favorites to win the third place.
Romania vs. Brazil

Final:
Serbia and Poland already met eight times before this, at the U17 World Cup (fourth, tenth - twice, twelfth, fifteenth, sixteenth, nineteenth, and twenty-fourth) - Serbia won five, Poland won three. They also played in the same qualification groups four times thus far (for the eleventh, fourteenth, eighteenth, and the twenty-fourth) - Poland won three of those, Serbia won two, and the other three ended with draws.
Serbia had an excellent semifinal match, defeating Brazil with 1-2. While Brazil scored first, Serbia managed to equalise and score the winning goal without much trouble. Even though Slovenia had quite a few more shots than Poland, Poland still won their game with 1-3 (Daniel Wisniewski scored a hattrick).
While Poland has a slightly better goalkeeper than Serbia, Serbia's team is a smidgeon better in the outfield, and with the two teams playing very different tactics, we are expecting an exciting final.
Serbia vs. Poland

3^rd Place Play-off:
Brazil (the 25^th U17 World Champions) and Slovenia met only once before at the U17 World Cup, and it was in the very first one (Slovenia won that one by the narrowest of margines). They did play in the same qualification groups twice thus far (for the seventeenth and nineteenth U17 World Cup) - Slovenia won all four of those (though again, usually just by a goal).
Brazil has the best goalkeeper at the U17 World Cup - Coelho. They also have one of the best (outfield) teams at the U17 World Cup. Unfortunately for Slovenia, they fall a bit short overall. Besides that, their goalie is not one of the better ones at this U17 World Cup. Brazil are the tentative favorites here.
Brazil vs. Slovenia