I picked up the Monster Hunter Tri demo yesterday, read the included "manual" at work, and popped it in when I got home.  After about an hour of sitting down with it, I still found it absolutely impenetrable.   There are items without apparant description, and classes the same.  I have no idea what I'm doing, or  how I'm supposed to be doing it.  
 
Can anyone offer any advice on how to explore/use this demo?  Because right now I can't imagine I'm getting a real impression of what the full game will be.

If you go to the in game menu and choose to view items then it should have a list of your items as 3 pages and text saying press a button for a description of the highlighted item. You can't check what they do if you're simply browsing through the item to be used. But the items are mostly self explanatory anyway. Potions for health, cooked meat (steaks?) for stamina, flashbombs are flashbombs, knives are throwing knives, torch is for dark areas, some meat I forget what kind is to paralyse the monster if you can get it to eat it, sharpening stones are to sharpen your weapon when it goes blunt (you get a pop up message to know, and your strikes will often deflect as well) etc. If it has first aid kits and rations (I forget) then they work as potions and meat respectively, though they're consumed much faster than eating. Other items you find may be useless in the demo, to be used for the creation of other stuff in the full game, but such items won't even be shown when you browse through the usables, and only show up in the item list through the menu as explained in the first sentence.

Most items in the demo aside from health potions and shaprening stones can be considered 100% optional, you can complete the hunts without using anything else, it's just to make it slightly easier that it gives you a healthy supply of items, and to check out a couple of functions like fishing that aren't really useful in the demo (though I think you can fish some fish that can be used as sharpening stones from that area). Also, paintballs are useless in the demo, they're normally thrown to track a fleeing monster in the mini map, but the demo has that icon enabled all the time.

What else are you having a problem with? A good newbie weapon is the sword and shield as it leaves you quite mobile and able to use items without sheathing your weapons first. Try to replicate this guy's tactics and it will click eventually, you'll play much better than him, aiming your strikes better, dodging all the attacks, and shield bashing it much more than he does causing more stuns. You can also maybe paralyse it if you get it to eat the meat, or if you throw enough paralysing knives at it, I forget what supplies it gives you, giving you some free seconds of attacks while it's immobile.

Glad you're enjoying it, and yes the long sword is really good. Well, all weapon types are after you understand their quirks. Just instead of only rolling out of the way as with the sword and shield, you need to use the slash dodges instead. It has various moves that do a slash and then a leap to left or right or back depending on how you did the move, you need to master that, feels good too. I dunno about finding players before the game is out, capcom unity perhaps? As for game information, what do you need to know? Look at past threads from myself and others in this board or ask me. This Unite thread may work too for general series information. Here's a video with the longsword beating the Jagi, similar to that sword and shield video.

I don't think it supports the GC controller (but I'm not sure). Just try to get used to the buttons, try messing around in the first area, first the basic controls of running around, dodging, choosing items, turning the camera, then unsheath your weapon and work with those controls similarly.

Try these vids too, they're for the Japanese release but shouldn't need translation.