5ed I think was the start of the need to field a good chunk of the points in your army for troop choices since scoring units became much more restricted and this has carried through to 6ed. 4ed for me was a relatively easy transition from 3ed with 3.5ed as a nice little stepping stone. I still remember in 3ed when the Blood Angels Death Company was a zero victory point scoring unit - it was pretty crazy.
I struggled in the beginning of 5ed and 6ed due to the major changes in the game. I'm not going to say I have mastered 6ed yet but I can certainly say I'm doing a lot better now than six months ago. One major change for me in how I play now was to accept some fundamental changes how I field my troop choices. There was a time not too long ago that I felt for whatever reason that my troop choices must be relatively costly rock hard units - this meant that I was paying a lot of points towards my scoring units for a relatively small number of units in my army... For example in the past I would have never fielded a five man squad of Necron Warriors.
Going into the local GT (the awesome Crucible 2) here in Orlando last weekend I was not happy with my army (Necrons-Grey Knights) and spent two whole evenings re-analyzing my list to decide what changes were necessary to be more competitve. Even with the popularity of the Heldrake I decided to focus on more infantry. I felt I needed a much more balanced army. I didn't face any armies with Heldrakes at the GT but then again I think the Necron Annilihation Barge is one of the best anti flyer units in the game.
Chaos Space Marines can field low cost cultist-zombie units for troops and I think this is one of the keys to the overall success of the codex up to this point in time. We have seen some hot lists for sure that take advantage of cultists-zombies. One of the problems with this type of list in a tournament is Typhus versus a Necron Overlord with Mindshackle scarabs... It sucks when he insta gibs himself. :'(
Now that the new daemon codex has been released I'd rather put the bulk of my points for my scoring units towards daemons. One of the great things about the new CSM codex is there are lots of options of your scoring units... I think fielding Abbadon so your Chosen score is really awesome.
Anyways those are some of my thoughts towards building a competitive list now for 40k. I'd like to see your opinions as well. : )