Can any race be any class? How do items work? Are there class-specific instances/quests?
1 - Yes. You can make a Baraka sorc or a Loli Lancer. No race/class restrictions.
2 - Weapons and armor are class specific. So, Warriors use leather, but Lancers can't equip leather at all (after a certain level e.g. level 10). Likewise, lancers can't equip the Warrior's dual swords. A really stupid thing IMO is currently you can't even preview the graphics of other classes' sets like you can in WoW.
There are potions and, yes, for once they are useful in an MMO. At least at level 30 they were still very useful, especially healing and mana. There are also potions to restore stamina but this is just as easily accomplished with a campfire.
There are scrolls of identification. Here is where my knowledge gets a bit fuzzy because I haven't paid much attention to this yet. But, when I played, scrolls "reveal" enchanting properties for items that can be unlocked through enchanting. Enigmatic items come in several different flavors requiring different types of scrolls. As far as I know, in the Korean version, you could burn all of your money trying to "identify" favorable stats (this was the biggest money sink). But I also heard that this may be changing in the American release, so take this with a grain of salt.
You have no hearthstone in Tera, but you have scrolls that serve similar functions (and sometimes racial abilities as well).
There are several what I'll call "flavor" items that may not make it into American release judging by what the devs have been saying. One of these is a Box of Transsomethingoranother Mischief, that can take an item like ore and potentially multiply it. 19 times out of 20 it just fucks off with the item so I never used them. Another, one that will probably stay, is an "egg" that can contain random stuff. They can be merged or split, I think, into varying levels of stuff.
Most, if not all items, particularly equippable items, are divided into tiers. This is relevant in enchanting, which I believe requires eating an item of the same tier (with better/more reliable results if the item is of significantly quality). I heard they're introducing Bind on Pickup items, which is a serious con in my eyes (I hate binding systems entirely), but otherwise green and above is BoE. Stupid. I preferred Lineage 2's way of doing items.
Probably some more item related stuff I forgot.
3. Not that I've seen yet but the first major dungeon can be 2manned with a Berserk and a Lancer (as shown in my video) @ around level 20. I personally don't care for class specific stuff so that's not a big down in my eyes.
Show them your butt, and when you do, slap it so it creates a sound akin to a chorus of screaming spider monkeys flogging a chime with cacti. Only then can you find your destiny at the tip of the shaft.