Synergies.

[Brista]

Short answer: cross-tree

Longer: As a player I would hope to see some subtleties of skill synergy interaction. I think that the Druid elemental tree was probably the skilltab done first and in many ways suffers from a lack of a full appreciation of the range of possibilities

I'm just guessing that the Druid Elemental tree was first, but the deficiencies previously in that tree demanded some solution and skill synergies are actually, I think, a pretty elegant fix to the problem of how to give the Druid a skill that works by doing X amount of damage that is effective in Hell that don't overpower late Normal and Nightmare

But what they've given us is big Fire skill multiplies other big Fire skill OR big Cold skill multiplies other big Cold skill

For players needing to get a lot of bang out of their choice of skills in order to complete the game or feel satisfied with their character this basically steers you towards two builds:
1) big fire spell, supported by a sprinkling of minions and synergistic fire skills
or
2) big cold spell, supported by a sprinkling of minions and synergistic cold skills

That's a fairly limited amount of replayability

There's not even replayability in terms of developing a different fire skill next time round since you can certainly develop two or three fire skills that interest you and try them all out with the one build

Now the Barb skill synergies are better. Pairing off various combat skills with various war cries is quite interesting and will give rise to a number of different builds

So, for instance if you did the following pairings with the Sorc skills:
Energy shield - Chilling Armour
Enchant - Shiver Armour
Teleport - Frozen Armour

you give players planning builds much more food for thought

A more sophisticated variation might be to loop it around, eg:
Energy Shield boosts Chilling Armour
Chilling Armour boosts Teleport
Teleport boosts Frozen Armour
Frozen Armour boosts Enchant
Enchant boosts Energy Shield

I would suggest that you basically balance most skills without relying on damage bonus synergies, except for a very basic Druid Elemental and Nec Poison and Bone should require some sort of "Mastery" type investment so that they don't come into full bloom before level 50. Use the synergies to promote interesting new builds rather and to encourage the use of under-played skills

I suggest that in general you pair strong skills with weak ones and medium ones with medium ones

Also try to imagine playing the character - Berserk (sets defence to zero) and Shout (boosts defence) for instance is a rather odd pairing