First was Space Marines, who almost look more like a 4th ed codex then a 5th one. They brought cheap transports which helped shift the game toward a more mechanized heavy game. They weren't too overpowered, though the older books did have to adjust to them. All in all though they were largely the same army, but needed to bring more marines to do the same thing for some reason.
After that came IG and there's where things start to break down. They fixed the issues they had with overpriced weapons and way too many Kill Points by making their weapons insanely cheap and allowing them to bring an entire motorpool at 1500 points. Then the damn list that I refuse to name (but it was named after a yard tool used often in the fall) and the world came to an end. IG became the instant-win army and people were pissed. People refused to play against IG because they expect that list. Add to it people exploiting the allies rules and you had something that really screwed people up and older books that had to really reach for an answer.
Space Wolves. Honestly I think Wolves are pretty balanced, too. They have great abilities, but pay for them (except for Jaws of the World Wolf, but they needed something insane, right?) They did some things right, but you get the first taste of Space Marine+ here. They do more then regular Marines at a better price. They did step up things but no where near what IG did.
Then came Tyranids. This was a BALANCED army and the bug players flipped out. "you mean we aren't insanely overpowered like the IG are? NOT FAIR!" Followed by some moron somewhere going "Oh the Doom can attack stuff in transports...nevermind the fact that they made this up and NOTHING IN THE GAME can do that. We can now... because we say so" Then GW ended that for the few that thought that was up for grabs with a FAQ. Sure they were wrong in the Shadows in the Warp, but hey, I play Chaos Marines so be happy you have some sort of anti-psyker. They got some beastly monsters, but have almost no invulnerable saves so that's balanced I think. What do you know? GW still remembered how to make something balanced.
They were followed by Blood Angels and again things were thrown into upheval. Now you can have FnP, Fearless, Furious Charge Marines in a fast skimmer assault vehicle that's melta-immune and oh yeah, it's carrying a DREADNOUGHT too! GW then came out with so many details on it and a beautiful model...wait... no they didn't. Now we have to figure out why that Land Raider is on a flying base. This is what Marines should be and I expect to see many a marine with a red paint job over its old blue one. They do everything better and often cheaper. When they're not cheaper, its for a very good reason. They aren't as bad as IG, but can still be broken pretty easily.
So now supposedly the Dark Eldar are on the way and with them, maybe we'll see another mostly balanced book, or something so disgusting it will make the guardsmen cry in their Chimeras. Or (just because it's GW) They'll get pushed back so an army that REALLY needs a book to become competative again (Necrons...Tau...). But if they do get pushed back it will probably be for Eldar or someone else that's holding their own in the new rules.
I look at the last few years of 5th edition and compare it to the recently released 8th edition Rockethammer. The day before the rules hit, everyone gets a FAq to bring some older books up to date and explain some of what doesn't work anymore. We have yet to see a new Armybook for the new rules yet so who knows if they'll be back to the old creep or launch something insane beyond all human comprehension but the next few months could be very interesting.