I'm not a basketball expert, I just love St. John's and our kids, so take this with a grain of salt. I'm gonna pick the one biggest thing I believe each player needs to work on.
Harkless - Certainly didn't expect this kind of production right away. Sure he could work on his shot, or add some more moves to his offensive game, but I think the most important thing for him to do this off-season is put on about 15 pounds of muscle.
Harrison - A more consistent shot would be nice, as would about 2 inches of height and 8 more inches on his vertical, but I'll go for a more realistic option of having better shot selection. Some of it has been forced upon him because of the lack of developed offensive talent around him, but there are times where he just makes you scratch your head. This might be naturally rectified next season with new players and improved returning players.
Greene - Greene was thrust into a position that wasn't expected of him and has performed admirably given the circumstances. Next year he'll be able to go back to being a combo guard, but I think his highest ceiling might actually be as a point. That being said, I'll say improved court vision is his biggest need. He needs to stop getting his pocket picked from behind.
Pointer - Defensive stud. Offensive wart. While obviously a shot would be nice, I think the first thing he needs to work on is improving his handle. He's never going to be a 4, and if he wants to see significant minutes at the 3, he needs to make us not afraid of him dribbling.
Achiuwa - At the start of the season, he looked great (albeit against weaker competition for the most part). He shows flashes, but overall seems to be a little lost and/or overwhelmed. He had hands and a shot and the beginning of the season; I'm not sure where they went. I don't expect greatness out of him, but I think if he can work on his confidence, it could go a long ways towards improving his overall production.
Garrett - Not as much of a body of work to critique, and it's hard to tell where he'll be at the end of the season given how much improvement he has shown. Plays hard all the time on both ends (now that he's not completely lost offensively). Hard to criticize his game as I think he may unknowingly (to myself) be turning into one of my favorite players on the team. I'll go with an improved shot; more out of team necessity than anything else.
It makes me giddy to think how much improvement we could possibly see next year if the players work hard. Can't wait.