Mulligan strategy for a burst Shaman

Are there some common rules on which cards to discard immediately?

That's pretty hard to say because it really depends on your build and playstyle. But the cards that cost 5-10 mana should in most cases be removed without thinking. Unless they're some really powerful high quality cards in which case it could be smart to keep one but only if your other cards seem good.

I think it's not worth risking early game by keeping high cost cards unless you're sure you can pull it off.

Besides the mana cost, which factors should I take into account making this decision?

Spell cards:

Some spell cards are more effective late game, even if they cost low amount of mana. Rather remove them unless you feel like they might come in handy early game. Keep good early game spells, especially those that cost low amount of mana and are able to destroy enemy monsters.

Minion cards:

I'd suggest keeping good taunt cards that are up to 5 mana cost. Taunts are very useful and you never know will you get one when you'll really need him later.

Low cost minions that let you draw more cards in any way should be kept as well. Quite useful.

In the end there's no magical formula which will tell you what to keep since you can't know what you're going to get next by drawing. Sometimes you remove cards you don't need early game just to get them replaced with other un-needed cards.

There aren't that many spell cards right now that take minion placement into consideration:

Spells:

• Betrayal - An enemy minion deals its damage to the minions next to it.
• Cone of Cold - Freeze a minion and the minions next to it, and deal 1 damage to them.
• Explosive Shot - Deal 5 damage to a minion and 2 damage to adjacent ones.

Obviously, most of these are highly situational and its up to you if you want to allow 2 high value targets to take damage, or potentially lose a low value target instead.

The only one that really matters is Betrayal, where you don't want to place high attack minions next to each other to minimize damage. Like you said, Faerie Dragon is a great mid position card as is any other stealth creature. If you can force them to target an end position card, that's great but the likelihood of being able to maintain something like that for the entire game is very low.

Also, here are all the creature cards that boost adjacent minions:

• Flametongue Totem - Adjacent minions have +2 Attack.
• Dire Wolf Alpha - Adjacent minions have +1 Attack.
• Sunfury Protector - Battlecry: Give adjacent minions Taunt.
• Ancient Mage - Battlecry: Give adjacent minions Spell Damage +1.
• Defender of Argus - Battlecry: Give adjacent minions +1/+1 and Taunt.