I have been to 8 games this year (opening night and several from January to last weekend) and spent most of the time prospect watching. I am by no means a professional scout but here is what I noticed:
Gryz- Most offensive minded of the prospect D, small, but generally in the right place when on the defense.
Casto-He is all around solid. He was (to me) the best d-man on the ice the first couple of games I saw. Something almost always happened when he would start a play from behind the net to start a rush.
O'Gara-He grew a lot from the first game I saw him and was doing 1st line everything the last couple of games. He made a really pretty goal one of the games and is seemingly never rattled. I think he is the most ready to make the jump but both the other two deserve at least a look
Subban- I can't stand him. He is either very good or gawd awful. I have been to one to many of the later.
McIntyre- He should be the back-up in Boston next year and pushing Rask for 1 and 1a minutes like Rask did with Thomas. He makes amazing saves and makes them look easy.
DeBrusk- Another story of amazing growth. The first game he would hang out around the blue line and wait to go on offense. Now he can take over the play offensively and is backchecking. Along with the good for him is the bad, he makes the occasional Pasta-like pass expecting his teammates to keep rushing and causing a turnover. I would seriously consider him for the Marchand development cycle. Throw him on the 4th line, get him some experience with the game feel and let him move up as warranted.
Again, just a fan but thats what I saw