Synergies.

[whyBish]

I have not tested myself, so I trully cannot say, but it has been said numerous times, and by numerous people, that skills are no longer viable on their own, for the most part. That, in fact, it is a necessity (hence where I got the idea from) to max, or at least invest heavily, in not only a primary skill (or skills), but also one, or even several of its synergy skills, as well.

The problem here is skills that do damage. Balance of skills and synergies is a lot simpler to achieve with non-damage skills.

For damage skills you have the following issues at a minimum to consider:
- sorc pretty much has only skills that differ in method of damage delivery. Once they have invested 20 points in that skill they can only increase their damage through synergy (I count mastery as a synergy). My pref here would be to re-do sorc skills so that they aren't all just different damage skills. Sticking with the current trees however, synergies (+masteries) are pretty much required to avoid the I am at full damage capacity at level X problem. This does however lead to the problem of forcing investment heavily in one tree (i.e. skill + its synergies). A real blunt 'fix' would be to have all masteries affect all trees, and have no other synergy. This way you could have a couple of primary skills maxed on different trees without being forced into a further skill just to boost one of those primary skills. (Note this idea is not fleshed out, e.g. lit/fire mastery would need to be rebalanced perhaps lit mastery returning to mana cost reduction or other)
- Physical attackers can upgrade their damage through multiple means. Most of these are multiplicative. An example is the amazon:
Upgrade weapon damage (on weapon, on active skill(e.g. strafe), on passive skill (e.g. crit strike))
Upgrade chance to hit (on weapon/equip, on active skill, on passive skill (penetrate))
Upgrade attack speed (on weapon/equip, on active skill (non-zon e.g. frenzy/fanat))
Upgrade (potential) number of hits per skill use (active skill (strafe/MS), pierce)
Reduce time not attacking (through skills such as vigor, minion skills/hireling, defensive skills and equipment)

Summary:
- Caster v.s. fighter issue: caster essentially needs to need synergies otherwise they are maxxed out at level X
- Fighter issue: how to control multipliers?

This is essentially where I see diminishing returns fitting in. Make offensive skills (methods of damage delivery) not be essential to max, make them inherently diminishing (examples to follow). Why? This allows pumping of other 'passives' that multiply all offence skills so that you can have a number of different offence skills used (i.e. no one trick ponies).
Examples, again the amazon. Strafe is essentially diminishing (has a cap on number of extra arrows) if you ignore the damage %. Same for zeal. After the initial levels the later levels should offer some alternative to damage (e.g. +ar%, boring I know but I'm sure there are plenty of other options, e.g. adding stun or other novelties etc.)