Actually, on Venos, you can build for light or even heavy armor without being short on magic at all.
But to do this, you MUST find ornaments (and until 70, armor helps as well) with stacked or multiple adds.
My stats (wearing only oraments/hat/robe) at 71 are:
STR - 78
VIT - 42
DEX -79
MAG - 210
My armor (light) requires 74 dex and str, and my weapon requires 210 mag. All are up to my lvl, I'm not behind on anything (unlike what many in the forums believe, you do not have to settle for armor or weapons lower than your level, unless you are building heavy and cannot find the right equipment).
Of course, you will only be able to achieve a build like this with excellent adds on your ornaments, cape, and hat.
My total added points (NOT including TT70 armor adds or weapon adds) are as follows:
MAG +10
STR + 8
DEX +10
VIT +11
For a total of 39 extra stat points, not including what my armor gives since I carry both a full light and full arcane set. These points can be greatly increased with a good tome, if one is willing to sacrifice a Blessing of Christmas. For now I will stick to not having to feed my pets. ^^
Heavy build is harder, but it's not impossible. It simply means only 5 vit, 39 dex (at my lvl, at least) and the rest in str and mag, with focus on str and mag adds. With the exception of TT and legendary armor, you can get armor with lower requirements than normal, making it possible to carry armor and weapons of the right lvl.
Though I do not have a BM or Barb, I know that stat adds can also make or break those classes, at least in PvP. A barb with +dex and/or +accuracy is much more deadly. Stats can also help you use more than one type of weapon for BMs, without sacrificing strength for too much dex, or sacrificing HP by having super low vitality.
Also, for any class, Citrine shards are your BEST FRIENDS. +HP adds help a lot, as well. If you are a squishy class (wiz, cleric, arcane veno) try to get +phys resists adds on armor (stacked preferably, like 2x +69 or 90) and socket with HP shards. Evasion is good as well, if you can get it as a bonus with any of the above.
Look in AH for good deals on non-TT armor and ornaments. People sometimes let things go for just above NPC price. Some may complain about spending a lot on ornaments, but it is MORE than worth it if it improves your build.
Also, socket everything you can with at least 3 sockets. 6 pieces of armor (inc. robe and hat/helm) x 3 sockets each = 18 sockets. Fill each one with at least an average citrine and you get +450 HP, lots more if you use higher grade shards or add the maximum 4 sockets. In a tight spot, those 450+ points can mean life or death.
So those are my observations/experiences/opinions. Builds vary greatly from person to person depending on how you play. I tend to take as many hits as my pet (multiple mobs, lol) so it helps a lot to have light armor. It definitely helps in PvP.
... this was a lot longer than intended.