Novak Djokovic: By winning the Australian Open, Djokovic can send a clear message to the rest of the tour that his slam "struggles" are behind him and he's still a level above everybody else. As good as he's been the last few years, Djokovic knows he should have a few more grand slam titles in his bag. With Nadal unlikely to be at his best, Djokovic has to believe he has a great chance to take home the title. He has a decent draw, the surface favors him and it's his most consistent grand slam tournament. Anything short of the title will be a big disappointment for Djokovic, and will create more questions (fair or not) about why he keeps letting major titles slip away when he's clearly the best player.
Roger Federer: Federer enters the tournament playing as well as anybody, and is certainly capable of winning his 18th grand slam title. His chances will be better if he doesn't have to play Nadal in the semis, though no one would be shocked if Nadal doesn't even make it that far. Winning the title will officially make Federer's latest comeback complete, and would arguably be one of his more impressive grand slam titles. Federer knows he let a great chance slip away at the US Open, when all that was left in front of him was Cilic and Nishikori. In a sense, Federer is in the part of his career where everything he accomplishes is just a bonus, so it's hard to say a loss would be devastating. He may not be levels above the field like he once was, but Federer would love to show the tour that at age 33 he is still the man to beat.
Rafael Nadal: None of the top players have more questions surrounding them than Nadal, and it's hard to imagine Nadal has a whole lot of confidence heading into the tournament. He lost his only match of the year to world number 127 Michael Berrer, and the end of his 2014 was no better. Given his lack of match play over the last few months (he had to shut his 2014 season down early) and dip in form, Nadal has probably lowered his expectations for the Australian Open. No one doubts Nadal's ability to come back strong from a long layoff or injury, but winning the Australian Open may be asking too much too soon. The tournament seems to be arriving before Nadal is truly ready, and fans shouldn't read too much into a poor result.
Stan Wawrinka: The defending champion would love to show people his 2014 title was no fluke, and he seems to have moved past his slump from the second half of 2014. Winning his second slam title would greatly change how he is perceived, as no one could say he was a one-slam wonder. No one should criticize him if he goes down to Djokovic in the semis, or even Nishikori in the quarters. However, a loss before then would send his ranking spiraling downwards and raise the question of whether he is still a top 5 caliber player.
Kei Nishikori: Nishikori reached the finals of the last grand slam tournament he played, and is good enough to take the next step. A title would get rid of any doubts that he has reached the highest level of the game. He didn't put up much of a fight against Cilic in the US Open finals, but shouldn't be as overwhelmed next time he reaches a grand slam final. He knows he is capable of getting through Wawrinka and Djokovic since he did it at the US Open, but doing it in back to back majors will be no easy task. A loss before the semis would be minor setback, but shouldn't have a long-lasting impact.