I do tend to agree with Wulf that 5th level spells are the huge gleaming sign post of arriving magically. Those are the big deal effects of fantasy: Turn someone into a newt, bring someone back from the dead, appear out of nowhere, send an illusionary image to speak on your behalf, etc...etc..
As a sign post, I actually think 5th level spells are more significant than any other level. My group is approaching 17th level, so that may change, as 9th level spells rear their head, but in a group w/o a Wizard or Cleric the feel of 9th level spells I am sure will be different.
Which brings me to my point: party size and party composition effect a great deal, what 'epoch' of D&D you are playing. My own group has consistently maintained a roster of 7-9 players. With that many people, I think you tend to have a shorter low level period, as with some gentle care and good tactics, the group can have a sizable impact on 'mid level' monsters.
Likewise...a group made up of pure melee, or multi-classed spell casters are going to keep the mid level balance a bit longer, at least until "THE THIRD ITERATIVE ATTACK".
That 3rd attack is a sea change event, possibly greater, but more subtle than 5th level spells.