Up until 2 updates ago, buying the affinity trait (Bowmanship, Pikesmanship, etc.) would allow you to use severely nerfed versions of class-specific abilities; Speed Surge lasted for 15sec and had a ~6min recast, Tranquility gave back 5% of MP with 20min recast, etc. This was patched out.
As for must have skills, there are certain skills that are vital on any job (particularly solo):
Shock Spikes (best low-level defensive spell)
Feint is another ability that is crucial for any DD (not so much for GLA or DoM). Sacrifice is actually a horrible spell for DoW, especially when solo; the 54 HP it costs to cast nullifies much of the gain when you can't AoE it. Cure is far more valuable.
For DoW, and ARC in particular, Skull Sunder and Trammel are two WSes you should get ASAP. They both share four things in common:
1) fairly short recast (~10s)
2) medium-low TP cost (500)
3) medium-low stamina usage (a little less than Light Shot)
4) single-target (note: Skull Sunder can be AoE'd on MRD)
ARC, in particular, has a particular problem with WS selection. Every ARC WS in the game has at least two of the following problematic attributes:
1) high stamina cost (>=40% of stamina bar)
2) long recast (>30sec)
3) high TP cost (1000+)
Skull Sunder and Trammel work great as TP spam options. I open every fight with a triple-nocked Light Shot followed by Invigorate, so I generally have a lot of TP to work with, but limited ways to use it. Skull Sunder and Trammel solve this problem. The damage is reduced a bit (because of affinity penalty) but they are still very efficient from a TP/damage perspective, and they don't rape my stamina bar.
Edited, Dec 17th 2010 11:45am by redvenomweb
Terraxia - RNG/COR/THF - Midgardsormr/Quetzalcoatl (boxed)
Viper Beam - ARC/CRP - Fabul