tl;dr - CSM 3rd Edition had a lot more fluff and options that I think should be encouraged and spread to other armies, as opposed to being obliterated as it was by their 4th Edition "update".
(In response to
Pete's Firewasp's comment in this post, I got a little preachy.)
I guess I didn't include Black Templars or Dark Angels. I only included Space Wolves because they have their own section outside of the Space Marine section on the website, and I imagine Blood Angels will too shortly.
CSM did get a fairly recent update, but that's a touchy subject with me atm. I like Nurgle. I like the fluff, especially the irony of it, and I like the toughness of the units. I like a lot about it.
When I bought my Tau, my roommate wanted me to go CSM instead. I got bored the other day and started to check out the CSM codex. It seemed kind of cool. Then I read some more about Death Guard. I was liking the challenge of creating an effective, fluffy nurgle army that didn't have a bunch of identical units.
I decided to look around online to see what others had found. What I discovered was a LOT of rage. This led me to find a copy of the 3rd edition CSM book. This led me to rage.
The Tyranid and Tau 4th Edition books are sort of sparce. They're thin books, lacking variety you find in the 5th edition books. It's hard to customize a Tau army in a functional way that is also effective. These new Tyranids...I can see 5 different unique, flavorful, effective armies easily. Same with Space Marines, Space Wolves, and Orks.
The 3rd Edition CSM book is awesome. Options all over the place. I think every army should have some version of Veteran Skills (Essentially any character or individual unit can be given things like Furious Charge, Counter Attack, etc. for a points cost) and that it should be extended to stat line modifiers as well (+1S, +1WS, etc.), but that's a whole different story. You could dedicate anything and everything, but everything had a drawback too that matched what it gained. Nurgle Fast Attack was limited to 0-1 Bikers and could only have 2 dedicated Rhinos as Troops, then they became Fast Attack.
This is all coming from a builder. My favorite aspect of the game is designing an army. I like themes and cohesive ideas that bind it all together. If there were more drones or more stealth units, I'd give up every Railgun and spend days finding a way to make it work. If I can't find a good story in my head, I loose interest VERY QUICKLY.
The 3rd Edition CSM codex is exactly what I've always thought every other codex should be like. Fluffy army related options with generic options that can be put together and too many ways for me to exhaust that would let me really make an army my own. My problem is really that they utterly destroyed that beautiful thing with an "update" that hunched the guts out and made what remained a little cheaper in points.
I'm just going to cut myself off and step down from my soap box here because it's almost 7pm and I'm still sitting in my office >.<