There will always be combos in games that are, if used correctly, out of balance with the rest of the game. The only way to make a game totally balanced is to give everyone the same equipment, like UT or Quake. If you want any amount of realism in a Fantasy game you need spells and equipment, and then there is already a balance problem around. (Unless you want standard spell effects, no special items, etc. Don't complain about how boring it is then though)
It is extremely hard to foresee all balance problems, only in heavy betatesting you will find them. People just don't all think the same, so something someone else didn't even consider (ab-)using may turn out to be one of the strongest features in a game. For those who remember: Master of Magic, War Trolls with flight, invisibility, magic immunity, eldritch blade and spell lock on them. Could not be hit ranged, with spells or by ground units, could not be seen by most flying units, would regenerate during combat and could hit monsters with damage immunity. I've finished games in the hardest mode using only 1 of those. Be glad they betatest it so extensively, rather than bringing out the game and then noticing a few lame assholes ruining the game for the rest on Battle.net.
P.S. Has anyone abused Sorcerer in Diablo II with a maxed out teleport and a ranged hitting hireling? Teleport gets cheaper with more levels in it, get a few skill boosts, and regen skills/items, and you can teleport around at an extremely high speed, far too fast for anyone to ever hit you. The hireling however will be able to fire the moment he/she/it reappears, and will be able to rock people hard that way. Never tried it on Battle.net, but I've used this during single player, and it works quite well. Don't accidently teleport next to something strong and you'll be fine
P.P.S. Also never got to try Druid with maxed raven skill + skill boosts on Battle.net, works fine on monsters. Summon a low level wolf to keep yourself from being hit or fired at, and keep summoning ravens. Their damage gets through all kind of immunities, and although it is not all that much, they can't be hit back, and therefor always will do their max.