1) Does it get more difficult? I am playing in Medium (whatever the default is) level and haven't come close to dying yet. Should I play at Hard? I noticed there are levels that are locked until I beat it in Hard mode.
2) Is the storyline the same for all character types? Or is there any kind of a variation for each class? Tank, Range, Healer, and Magic, seems like a straight forward setup for multi-player grind group.
3) There is a training ground you can access, but there is nothing there. Is that significant for anything later?
4) Are trades significant? Are they class specific?
5) Is there a message board dedicated to Diablo 3, more specific to the PS3 or console play in general?
1) The game can either be very easy or very hard depending on the mode you are playing
There are 8 modes of monster difficulty:
Easy, Medium, Hard, Master 1, Master 2, Master 3, Master 4, and Master 5
You are playing on the second easiest setting which means you are basically in god mode.
There are 4 modes of the game (see below). Each mode represents the same 4 acts with new and improved monsters to help you continue to level.
Normal, Nightmare, Hell, Inferno
I am a level 60 character(max before Paragon kicks in) and level 6 paragon. I am playing Master 1 difficulty at Inferno level now and it is challenging. I would not stay alive even a second in Master 3 difficulty when faced with a pack of elites.
2) the story is the same for each character
The characters themselves have a ton of variation. I completely changed my wizard late in the game to deal with end game stuff. The mixture of skills and runes can lead to numerous combination with in a character class, assuming you have turned on elective mode.
3) No training ground but you can replay any quest you have already completed if you want to practice, farm, or grind.
4) People do trade online. Some items are class specific, some are not. It will be defined in the items description if it class specific. End game appears to be very gear oriented.
5) I have found this message board helpful. It is trophy based but people answer plenty of regular game questions.
http://www.ps3trophies.org/forum/diablo-iii/
There is also a mountain of information on the PC version (which is very similar) here
http://us.battle.net/d3/en/forum/