Personally I find Guard OK to play in 5th: they get access to lots of Troop choices, units can no longer consolidate into combat, Battlecannons are more powerful and more accurate, better cover saves help a lot.
On the other hand, you might as well shake your opponent's hand in a kill point game, as a small Guard army will probably be putting down double the kill points of it's opponent.
However, despite them being generally very playable, I do agree that they need to be bought in line with some of the trends of 5th edition.
Also as others have pointed out, the original book wasn't perfect, it's just that we have got so used to seeing the flaws that we forget they are there. Probably less than half of the doctrines and army list are ever picked, and when the more marginal parts are, it is generally in exactly the same way. Also, you are never encouraged in Guard to take more than 25 men in an infantry platoon, because the command squad is so good and versatile, I think this absolutley sucks. If the new codex makes it viable to take 50-man platoons then that will be a very good thing.
I think the fact that we are getting a new codex, with new artwork, fluff units and an updated model range to go with it is a very very good thing. And there are certain trends in 5th edition that I think will suit guard hugley, like the emphisis being moved more towards numbers of basic guys being your killing power, rather than just being wounds for a static heavy weapon.
Also if the rumours about new guard infantry are true I think in 6 months time we will be looking back and laughing about the time when guardsmen were a joke!