Okay I stole this idea from OrdoBob so credit must go to him as my muse (though the concept of the article ended up being a bit different than our discussion). One thing which has always bugged me is people claiming armies are 'finesse armies.' Most notably this in reference to armies such as Dark Eldar and Eldar but you'll often see it tossed about for specific units as well. More often than not, it's just a smokescreen which can say any number of things. This sort of phrase is very similar to phrases such as "it works for me" and "you're just not using it right." Whilst these can certainly be true (stop driving your Chimeras backwards), it's more often a misnomer the speaker is just not willing to accept. In this regard we see the finesse army thrown around as though it requires a delicate touch to use which requires a completely different set of principles from other armies to work effectively. Walk through this smokescreen and you see it's generally not true.
We know the majority of the tactics of this game, yes there are tactics, revolve around movement. It's pretty simple when that phase sets up all your subsequent phases and turns and where you, as the controlling player, have near complete control over what happens. EVERY army needs to make use of the movement phase to its fullest or it will lose against an army of equal power level which is maximising its movement. You'll hear a lot of people complain how Imperial Guard, Space Wolves or Grey Knight armies are "point, move forward, shoot, win" armies. Against bad players and bad lists - yes, you can basically do this and do well and if you have two of these armies going up against each other with those sort of tactics, whoever's dice are best will win. You can see why people think armies and units which don't move forward are indeed 'finesse' and that the game isn't tactical.
When you start applying movement outside of "drive forward" to your armies and units however, you open up a whole new ballgame. AV12 Chimera walls look pretty weak if they keep pushing forward and you move...wait for it...laterally. Say hello to AV10 boys and girls. Again, apply the concept of threat ranges to some of the most potent guns in the game, meltaguns, and take this even further with the concept of optimal threat ranges and driving forward and jumping out every game just seems dumb. Unfortunately, the majority of armies out there currently can do this well to a certain degree. Any Marine based army is still pretty durable when overextending themselves and Imperial Guard have a lot of tanks and infantry models as well. What this means is you can use crap tactics (or none at all) such as "Drive Forward and shoot herapderp!" and still do well. You're not maximising the potential of your army but against sub-par armies like the majority of those from 4th edition, poorly designed lists or other generals not utilising the full extent of their movement, you'll still do reasonably well.
This is where the dissonance between these 'non-finesse' and 'finesse' armies comes in. The armies or units often labelled 'finesse' generally suck if you throw them at the enemy. Why? Because their points aren't being paid for the all-round decent statline of Marines or aren't cheap enough like Imperial Guard/Orks to simply suffer the consequences. No, you pay your points on such armies and units for extra tactical control. Whether it's the ability to deep-strike, infiltrate, outflank or come on any where you like or simply by having more options in terms of moving and shooting - these types of units and armies do not do well under the "drive forward and shoot" simplistic model of gaming. You have to actually know how to play the game and utilise the tactical options they give to you.
Now this may actually sound like finesse and in terms of dictionary definitions, well it's right. If I get the placement of a Rhino wrong by a couple of inches, it's still pretty durable. If I get the placement of a Raider wrong by a couple of inches, well it's a lot more fragile. In terms of actually executing my tactics and strategies then, the armies often labelled as 'finesse' need a certain amount of 'finesse' in their application. Colloquially though, as we saw above, the dissonance often comes from players using bad tactics/strategies with more robust armies which don't translate to more fragile armies. The term 'finesse' is then thrown about to cover their inability to explain how to use them or failure to do well with them (yes, that's a generalisation). The simple fact of the matter is, 'finesse' armies and units are using the exact same tactics one would employ when using the more robust and forgiving armies well - they are just extensions of this or often have more options all bundled up together.
For example - we have a bad Imperial Guard player who always overextends their Chimeras. This exposes their side armor and meltavets to countering fire. A Vanilla Marine army is going to utilise as much as their firepower as possible to get into that AV10 arc to cause maximum damage. Because they aren't a very fast army though, not everything can do this. Change this army to a Blood Angels army which has the same robust core as the Marines but also extra speed at extra cost. They'll have less guns overall but are more capable of getting them into the AV10 arcs of the Chimeras. Finally we have Dark Eldar who are even more mobile than Blood Angels and with more guns as they are not paying for durability. They are therefore most likely to cause the most damage in the AV10 arc as they have the most guns capable of shooting into said arcs.
This is a very basic concept (moving into weaker armor value arcs) and we can see how a 'finesse' army has more options in this regard because it has more control options during movement. If, on the other hand, it sped forward into the clutches of the Imperial Guard army, they would have taken the most damage being the weakest army of the examples used in terms of durability. This doesn't make it an army which needs a particular playstyle, though yes it plays differently to many armies out there. Rather, with such armies and units you need to ensure as much as possible you are maximising the abilities you pay for. Often this is movement, special deployments or ways to frustrate your enemy. The armies or units in question may be less durable because of this (or very expensive) which generally means they cannot be thrown at the enemy with a good game result to be expected.
Conclusion
No army is going to play the same. Trying to play Imperial Guard like Marines and Marines like Orks and Orks like Dark Eldar is going to make you lose. However, some armies are capable of just running at the enemy and not falling flat on their faces. Armies which are more heavily weighted in the movement department, aka the colloquially 'finesse' armies, are less capable of doing this and therefore require more thought into their basic strategies from the perspective that "drive forward and shoot" is how the game is played. If you don't prescribe to this understanding you'll find that armies which are unable to do this are no in fact 'finesse' armies but rather armies which apply movement and shooting constructs within 40k differently.
In essence, finesse armies do exist in the actual meaning of the word. They are perhaps a less than optimal choice for a newer player as they come to grips with the movement mechanics and strategies employed in this game but that isn't to say they require a new set of skills not normally seen in other armies. This is where the issue with the 'finesse' army or unit comes in - it's a cop-out and doesn't really explain everything.
In the end, these armies and units will require more thought to gain their fullest application certainly but are still based on constructs a good player should know from other armies. Simple tactics or lack of tactics will often see these armies or units blown away which has lead to the misnomer of finesse and the strategy/tactics required for them to do well don't apply to other armies. Apply their strengths within the tactical concept of 40k and they aren't a finesse army but one which plays a particular way within that construct rather than outside it.
