From about 7-9 you'll see a marked improvement in power/survivability, but nothing that can't be handled. Your characters are now like the beginning fellowship of the ring, they're real heroes. They can really shake a large town or even a small city with some planning and luck. They're no longer the people who need to seek out an adventure half the time.. they're the guys people start going to to for help.
From 10-14 you'll see the real power coming in, and fast. At these levels you need to constantly stay on top of your game. Your fighters will start becoming killzones on their full attacks. The casters will start getting their crazy powerful plotbreaker abilities. You'll need to start designing around both. Your characters WILL surprise you. And you'll likely start asking to see their sheets to work out precisely how they just did 150+ damage in one round or whatever. They'll start breaking your encounters easily and you'll need to readjust your expectations.
They're the people who kings and archmages might look to for help dealing with world level threats. They can even start dipping their toes in to planar threats. You're looking at the level of power of such characters as
From 15-20 you're really looking at people who are going to make legends that will last for centuries or even millennia. They're going to be responsible for things that will mark the world. At this level you're really dipping in to mythical levels of power and immortality is within reach if the characters are the sorts who will pursue it. Your fighters and barbarians are on the level of power of Druss the Legend, the rogues assassination skills are on par with Dakeyras the Waylander. Your Ranger can rub shoulders with Drizzt. Your Wizard is can likely match the magical might of characters like Belgarath the Eternal Man.
Consider houseruling vs scrying + teleportation for some plots. the enemies fortress might have anti teleport/scrying wards.. or maybe your worlds scrying spells create null zones where you can't teleport for 24 hours after the area has been scried on. that kinda thing.