"Balanced Party" is actually a really complex question.
I'd like to reword the question: How can we, in D&D 4th edition, create a party that can overcome challenging obstacles successfully?
In many ways, this is an exploration of the norming step of small group formation.
The first thing to note is that every player considers different things obstacles. I would like to direct the reader here in that there needs to be an explicit social contract outlining player motivations and what they want out of the world. (Doesn't have to be written down, but it should be fairly clear to everyone what people in the group desire.) Some people want butt-kicking, others want psychodrama. Without communication, an unsuccessful party can arise through lack of social cohesion.
Designing the Party
To create a coherent party, party creation must begin before character creation begins. What is the organizing force of the party? How does the in-world force cater to the players' desires for gameplay?
As part of this, and as part of the social contract, explore the alignment question: idealistic good is functionally incompatible with mercenary and chaotic unaligned. Someone won't be having fun. (Unless, of course, the player /wants/ player-versus-player conflict —usually with words — but this must be agreed upon in advance. Communication is key.)
Once you have the "group" created, the mechanical beast rears its head. Not all powers are created equal. Worse off, some powers "sound" awesome... and aren't. Players who don't memorize the DDI and CharOp boards will be worse off, mechanically, than those who do. The trick here is to have rough equality in power. The best way to battle choice paralysis is to present most players with "scaffolding." Go to the charOp boards and make a list of the blue and light blue powers that they can pick for their level. (Never ever start a party at high level. Learning a character is complex enough at 1-3.) Obviously, make recommendations, not requirements.
Another way to do this is to ignore the books entirely. Have people describe their optimal "heroes" down to their signature moves. (Everyone should have a good reason for adventuring with the group. I've been bitten by that problem myself a few times.) Allow the people who like mechanical bits to then flesh out the powers so defined, but for the rest, reflavour and retag good powers so they match the idiom described. The huge abstraction gap between mechanics and flavour is your friend, if embraced.
Everyone should, at 1st level, have +4 or +5 in their primary stat. D&D is, ultimately, a game about combat. Sucking at combat while everyone else doesn't is not much fun. As a GM, make sure the numbers work. Let the players make flavour, then instantiate the mechanics based on their requirements. Looking through the books should be encouraged for them to find awesome stuff that looks like fun, ignoring the mechanics.
This was the easy part. The hard part is getting the party to cooperate. Uncooperative parties in 4e are deadly to themselves and to fun. Pavlovian encouragement may be necessary at first. Hand out bonuses to people who both plan (and RP) their characters. Awesome should always be rewarded. Cooperation is awesome, involvement is awesome, using the environment is awesome (and should always either be a minor or be more effective than an at-will if a standard. Try to have at least one awesome environment thing for a fight, but this is a digression)
Getting past norming into a cooperating group is hard. And here is where I'd like to direct the reader. If everyone has a good to-hit and powers that aren't red or purple, then the deciding thing is tactics. As a GM, use effort-based XP to basically "auto-level" the enemies so that players eventually have a mix of easy and hard fights, regardless of what level they are.
Optional party generation trick:
After figuring out the reason for adventuring (never ever just meet in a tavern) have everyone, including the DM, write down a race on a blank sheet of paper. Pass the sheets around the circle. Then, everyone chooses a fun class for the race. Pass. Based on the race and class given to you, choose 2 backgrounds, pass. Then next person chooses 2 more backgrounds. Pile all x+1 characters in the center and everyone grabs one they like. The players then detail the particular class features they want and the rough "theme" of the character. Encourage them to imagine a moment of awesome. Then let the person who loves paperwork generate a short list of powers to choose from at each level.
Comments strongly encouraged as well as feedback.
I've used the party generation trick when running games of Pax Draconis many many years ago. It was highly successful. The rest of the discussion is based on landmines that have exploded in parties I've played with or GMed for.
Also, be aware of suicidal characters; it almost always means they want to swap out. Let them do so, because it'll just threaten the party otherwise.
On reflavouring, and its difficulties: link