Waterproof, there's a can of worms (or perhaps a bucket of eels)...
Let's start with the pack. Most packs are made of waterproof material, but that's not the same as a waterproof pack, because the material is joined at seams and they will generally leak. There are exceptions, with all the seams welded, but these are quite unusual (e.g., Ortlieb's packs) and unless you're quite sure you have such an exception then you want either a pack cover or a waterproof liner. I much prefer the latter, even in a waterproof pack, because if you have a waterproof pack and you put your wet overtrousers and jacket in when the rain stops then the waterproofing is just keeping all that water inside. If you have a liner then the dry stuff stays in that and any wet stuff, water bottles etc can go outside the liner but still in the pack.
Next, on to waterproofs. Generally the case that any day you spend in continuous rain you're going to get wet, because even if the rain is kept out there's perspiration from inside and it doesn't really have anywhere to go once the outside of the waterproof is wet through (that's not leaking, but it stops any vapour from perspiration getting out, and straps will stop that too).
Try it an an older non-breathable cag and you'll soon appreciate that some breathability is a lot better than none! You're in a small, closed system where keeping the outside water out makes it difficult (pretty much impossible over several hours) to deal with the perspiration being generated inside. The high humidity you'll encounter on a rainy day doesn't help: think of how much more windows mist up on rainy days.
You've other problems in all-day rain, which is any water getting in to your mid-layers around the big holes (sleeve cuffs, hem and neck) will wick through the inner garment and travel along the fibres. Over enough time this will make innards quite wet though the waterproof outer layer is not technically leaking. If that sounds bad then look at a paddler's or diver's dry suit and see what genuinely waterproof seals are like... you really don't want to walk up a hill with that sort of thing!
Pete.