Re: What is the Best Combination of Professions for
A good GM will pitch challenges suited to the party, because a good GM isn't trying to get the party killed, but trying to stretch the party and enhance their Awesome. Having said that, a good GM also isn't going to let the party get away with just anything. In a way, party composition is both vital and utterly unimportant dependent on a) what kind of GM you have, and b) whether you have any specified requirements for a challenge before going in.
What I mean is this: If a party is composed of nothing but spellcasters, pushing them into a challenge in which they can't use their magic at all isn't particularly cool. If, however, the heroes know they're headed to a challenge in which magic will be problematic at best, and don't bother taking anything but casters, that's their damage - and possibly their death certificate.
From the perspective of starting a game, or of acepting new players into an established game, the GM has the right (and, in some very real ways, the responsibility) to ensure that the mix of characters is going to be suited to their campaign. This means talking to prospective players about what mix you want. It means telling RTJs 'No, we have enough clerics, will you consider a fighter?' It means making clear (perhaps in a public RTJ post, the Game Info, the ad in the appropriate forum, or all three) what kind of character you're after. If the GM knows their campaign is going to require certain roles be filled and then does nothing to make sure that happens, their campaign isn't going to be fun for anyone.
I personally think that it's more helpful to think in terms of roles than in terms of classes. A lot of systems out there are flexible enough that more than one class can fulfil a particular role (spell damage, non-spell damage, tank, buff/debuff, healer, loremaster, social butterfly, etc). If the party has a broad, solid range of roles covered then they should be at least fairly okay.
And as Kessa says, creativity is key. The players may be able to swing some crazy stuff the GM hadn't even considered - as long as the GM is flexible enough to allow them to try.