81. it seems that the power level of characters (certainly at higher levels) has been reduced.
81a. is this true?
Warriors are stronger than before, casters are not.
All roles should be balanced at all levels. Also, extending the sweet spot.
81c. what effect does this have?
Generally a more balanced game. You don't have 1st-level wizards with only 1-3 spells battling alongside far superior warriors. On the other hand, you don't have 20th-level fighters whose primary purpose is to carry the wizard's water.
IMO the difference in power between level tiers in 4e is considerable, putting some stress on the DM (you basically need to learn to run three different games), but at least the PCs are generally balanced
with each other.
82. it seems that the power level of magic items (certainly at higher levels) has been reduced.
82a. is this true?
In comparison to 3e, yes.
To make PCs less dependent on magic items. Your non-magical abilities (especially AC) are far less dependent on items than on your class abilities.
82c. what effect does this have?
Third edition had the Big Six items. Amulet of Natural Armor, Magic Shield/Magic Armor, Magic Weapon, Cloak of Resistance, Ring of Protection, and stat-boosting items. These were the most efficient items in the game, and you were expected to have them. (The iconic PCs in Enemies and Allies all had them, for instance.)
This meant PCs would not spend money on "cool" items. If you got your hands on a Trident of Fish Command, you sold it and spend money on a more efficient weapon. Note how many of those items are based on defenses, since they scaled very poorly.
Also note that casters were less dependent on items than non-casters. (A caster need not spend a penny on a magical weapon, and if you're a wizard you have options that make your low AC irrelevant.) This created yet another balancing problem. And then there's the issue of slots. The poor monk wants a Periapt of Wisdom, a Cloak of Resistance, an Amulet of Mighty Fists and an Amulet of Natural Armor. You need all four items but they're competing for two slots.
Instead of the Big Six, in 4e there's the Big Three. Weapon/implement, cloak slot (Amulet of Protection, aka Cloak of Resistance Slot) and Magic Armor. All PCs use these three slots.
My current wizard PC wears "cloth armor" and bought a Robe of Scintillation (+1 cloth armor that has a daily daze attack) and wields a magic orb in place of a magic weapon. In the game where I'm a player, rather than DMing, we have a monk... who only needs to spend on those three slots, but at the same time has to spend on those three slots like everyone else. (For weapons, a monk can either transfer the bonuses to their unarmed attacks or buy a ki focus.)
The items scale at the rate of +1 per 5 levels, an easy metric. PCs generally only have enough wealth to afford three or four items of their level. So that's three of your level and enough to spend money on potions, rituals, etc. I haven't built a 4e PC beyond level 1 in a while, but it's easy. Build your PC, then spend almost no time picking three items. This is in contrast to the intricate process I went through building a brawler for Pathfinder's latest playtest.
It means magical bonuses are predictable, a big deal when trying to figure out what a PC's AC or non-AC defenses or even attack bonuses are. In fact, I generally don't have to look at my PCs' defenses; they're nearly impossible to break, so I can trust the monster math. (If I feel the PCs are steamrolling encounters, I can just add more monsters to later encounters.)
By contrast, the "typical" AC and saving throw values of a 3e or Pathfinder fighter is not predictable and you need to spend time looking at the character sheet... and that's assuming no buffs. They could have different values by buying different stat-boosting items, or different combinations of Rings of Protection, Magic Shield (if any), Amulet of Natural Armor, Magic Armor...
Inherent bonuses work in 4e, in a way they didn't in 3e. In 3e, a fighter equipped with a magic shield could have an AC 7 points higher than a fighter without a magic shield... and there's just no way inherent bonuses can cover both types of fighter in a balanced manner. I shudder to picture an inherent bonus monk in 3e (I know Exalted Monks tended to be broken). In 4e, a heavy shield gives you +2 AC, period. The math is tight enough that the +2 AC always makes a difference, but there's no dual-stacking to worry about. You can certainly buy a magic shield that gives properties (bonus to resist charge, bonuses when bull rushing, etc) but it won't boost your AC and it's not one of the "Big Three" so you don't have to buy one anyway.
When my group was playing 3.5 (rather than Pathfinder) flaming weapons and similar weapons were considered valuable. Instead of a +2 sword, I might get a +1 flaming sword. That's -1 to hit for +2.5 damage (often a worthwhile trade) and if I attacked a creature with non-magic DR, unless the creature was fire-resistant they were at least going to take that damage. (Replace fire with shock, thunder, whatever, if you worried about how common fire resistance was.)
In 4e, I don't have to decide between enhancement bonus and the type of sword. The enhancement bonus is simply based on the weapon's level (which is usually very close to my level). Of course, the flaming sword can only deal extra damage once per day. (At any time, I can make it deal fire instead of normal damage. Handy when facing an ice elemental, and I can just turn that off facing a fire elemental.) If I found a "trident of fish command" (and my PC used a trident) I wouldn't try to sell it off. It wouldn't "suck" for my PC, even if a vanguard trident were a bit better.