Taking a look at the current options for armour (which are still terribly designed), your AC probably goes up by 4 if you are in light armour, 1 if you are in medium armor and 2 if you are in heavy armour, from starting equipment to the best available. Given that the best attacks probably improve by 6 (+3+1 to +5+5), the intention there appears to be that you hit more frequently with level. The starting balance is such that you need about a 10 to hit.
Saving throws will start at around 14, where your best ability will give you +3 to save, so you need an 11 to save. As noted above, your best ability goes up by 2, whereas the saving throw DC goes up by 6. This is about the same as htiting heavy armour then, except that's the *best* case scenario - in reality you will probably never increase 2-3 of your abilities.
Firstly, is it ok that you get better at hitting (each other, didn't look at monster AC) as you increase in level? It does feel like progression that way, so I'm going to say yes. Thus it's ok for saving throws to become more difficult - except that there are 6 of them you need to consider, and they aren't at the same starting balance point either. We could put some load on the classes, as previous editions, and give saving throw bonuses by level. We could offer equipment that improves saving throws (difficult for mental saves if we're talking about mundane things). We could remove the bonus to saving throws, so they progress only by ability increase, but that might make spellcasters feel like they lack improvement. I'm stumped - there's not really an elegant solution without some proper reworking of the system.