Some cards that are great to start crafting: antique healbot, Big Game Hunter, annoy-o-tron, knife juggler, wild pyromancer(great for pally), Azure drake, Sludge belcher(amazing taunt), molten giant.
I'd advise against crafting commons. You go through so many packs and most of the time they're 4 commons 1 rare. Eventually you just have all the common cards naturally. You really want to save dust for when you need a single legendary or a couple of epics to finish a deck you're building.
I think it's also a common beginner's mistake to randomly craft plain good cards without the decks to put them into. For instance, Healbot is great but right now it's not really what he needs. It's not going to help him with his board control Mage or his eventual Zoo/Face Hunter decks.
I think the strategy most beginners use is: play arena/do daily quests, open packs, build a cheap deck like Zoo or Face Hunter based on what you've opened so far, use those while working towards building a higher cost deck, continue from there.
@machofantastico: Definitely put in those Sorcerer's Apprentices in place of the Bloodfen Raptors. They're just strictly better. Quick tip: Sorcerer's Apprentices do stack and CAN make spells go to zero. So if you have those in your hand plus Arcane Missiles make sure to play the Apprentice first.
Not sure if you've gotten any GVG packs from arena but Spider Tank should be a good replacement for Razorfen Hunter. Also, Piloted Shredder(also from GVG packs) is a good replacement for Chillwind Yeti. Don't bother crafting them if you don't have them. They're both commons and you'll eventually get them in a pack.
The single biggest mistake I made early on was recklessly disenchanting stuff. I'd say, if you're really planning on playing HS for a while and you want to eventually get good and build up a competitive collection, then don't disenchant anything for now.
If you go into Crafting Mode there's a button in the lower right corner labeled, "Disenchant Extra Cards." You can't put more than 2 copies of a card into a deck so this button disenchants those extra cards you don't need.
I disenchanted 2 of the best legendaries in the game(Alexstrasza and Black Knight) within the first few months of playing to craft crappy cards that I never use and know now are total garbage.(Pit lords and Felguards)
I feel like a properly designed boss encounter in a card game should be beatable by any class given the right deck construction, but It doesn't feel that way.
I'm fairly certain that's the case for the normal difficulty. Not sure there are any unbeatable bosses there for a particular class. But I feel like if they were to attempt that for the heroics, every boss would feel very samey and no one encounter would be able to reach any great highs. Every one would need to be brought down to a certain baseline. It'd be too constrictive. And maybe I only think that because the game has only been out for a year and each class is still fairly unique--you can't do everything with every class. When more cards get released and each class has more options it will probably be possible to return and beat all the heroics with all the classes.
@zevvion: Yeah, what is the time from getting into a game to passing turn one and then being able to actually play the game for that challenge? Like a minute+? They really need a quick retry and a "squelch all AI voice" option.