I just played with the 2nd edition rules. Now I remember why I was so glad 3rd edition came out. What an overly complicated, frustrating mess that game was. There were so many different places to reference stuff, no tables with the weapon stats all listed.
What a freaking disaster. Anyway I'll post a battle report shortly.
For troops, 2E was fine and had some great ideas and rules. But bigger games and vehicles were horrible to play with - that's partly why Necromunda worked better I think. As I said, I'd probably lose the 2E vehicle rules and go with something like the later editions used.
I personally think that 3.5E was my favourite version - it incorporated all the Chapter Approved amendments and updates into 3E, and was still fast enough to get done with in a reasonable time. It also got rid of all the random dice you needed for the various weapons and the thirty different templates, which meant a lot less equipment was needed. I felt that psychic powers were rather bland though, and would have benefited from a WHFB style magic phase with power dice.
I think most of my friends liked 4E a lot, not least because all the armies got a bit more detail but the game still retained most of its streamlined play from 3E. I found the extra detail was uneeded, but the armies were better balanced oveall (Codex Space Marines and Codex Chaos Space Marines 3.5 notwithstanding). I think this was the edition that made assault cannons rather OP too (they were better at penetrating armour than lascannons and still great at smearing troops too!), and I remember seeing SM lists with something silly like 20 assault cannons in them!
Going back through the different editions is interesting though, because each one had a different emphasis and different balance. It also highlights a level of frustration I remember experiencing in that every version would fix one thing and break something else that used to work fine.
I reckon the main reason I liked 2E and 3E most is because the style of games I liked best were something like 40 models a side.
Anyway, enough waffle! I look forward to reading your report. B)