The way I perceive encounter difficulty is based more on this method of viewing combats. If the party hardly expends any encounter powers and only one or two healing surges across the entire party, then that was an easy encounter. If the party had to expend a few healing surges each and used all or most of their encounter powers, then that was about average. And if they had to expend all their action points, encounter powers, daily powers and item powers and burned through several healing surges each, then that was a near TPK, 'cause if it had gone on much longer, they wouldn't have had any resources at all to draw on.
How do you perceive encounter difficulty?
Encounter difficulty is situationally dependent and it is very dependent on the tactical abilities of the players and the capabilities of the PCs.
I had what I considered a fairly difficult encounter yesterday as the first encounter.
The five second level PCs had in the previous session wiped out everything in an underground "bunker" below a fallen tower (long story, won't bother you with the details). They had had to lower a rope down into a pit that led to the bunker, but after an extended rest, they got back and their rope was gone. The room they were in was 10 feet tall and the sides of the pit leading down to the room were 20 feet and in the center of the room. So, they had to lift PCs up so that they could climb up. The sides of the pit were hard packed dirt and vertical, so a pretty difficult climb because the dirt could pull away.
The first PC out of the pit was going to lower a rope, but was ambushed by 4 necrotic werewolves (1 Frenzied Brute 6th level, 3 refluffed Wererat Skirmishers 3rd level), the monsters that had taken the rope.
I expected this to be a tough fight. The encounter was N+1, but the pit would split up the party and could/should have made it tougher. I knew that the Eladrin could teleport up and bring someone with him, but the first PC was going to be bloodied after the ambush (and that's what happened) and the other two PCs were going to still be in the room below. I expected a 4 on 3 fight with the monsters regen-ing, the PCs in the pit having at least a few rounds before they managed to climb out (with some possible falls back into the pit and taking damage), and the bloodied PC being in serious trouble in round one.
Instead, the Eladrin teleported up and the PCs started pushing/sliding the werewolves into the pit, starting with the high level brute. So, everyone got to participate right away and even with regen, the PCs wiped out the entire encounter in about 3 rounds. The first ambushed bloodied PC used up several healing surges, but most of the rest of the PCs used up maybe 1 each.
I see this a lot in 4E. Combats are often pretty easy unless the DM's dice get real hot and the player's dice get real cold. These are only second level PCs, but there is already no real good way for myself as DM to take into account all of the possible synergies of the PC's abilities. Yes, one could say that I should have known that the PCs could slide monsters into the pit, but as a general rule, most players know their PC better than the DM does (and in our case, we have 6 fairly consistent players and 3 other players that may or may not show up, so there are 9 PCs for me to be familiar with).
The other thing that I've noticed is that as the PCs get higher and higher level, it starts becoming more and more difficult for the DM to challenge them. Our DM typically had to throw N+3 or higher encounters at us, or have a lot of detrimental effects in an encounter to challenge our high Paragon level PCs over the summer. There were just too many ways to lock down a foe (turning the toughest one into a toad, or sending it to the feywild, or whatever), or have multiple strikers take a foe out each round, or putting up walls to slow up monsters, or whatever at higher levels.
It's not that high level PCs do not get the snot kicked out of them. They sometimes do. But, the players are not really concerned about it like at low level. The monsters do less relative damage and there are just too many options to get a downed PC (unconscious or stunned or paralyzed) back into a fight, and there are just too many go to the well abilities. Additionally, there are so many ways to get temporary hit points, damage resistance, and surgeless healing that PCs do not even go through healing surges as fast at higher levels.
And the mathematical reason for this is pretty simple, even if the DM always makes sure that he uses last year's higher damage for all monsters.
Same level monster damage single target ~= level +8.
PC hit points range from CON+10+(level-1*4) to CON+17+(level-1*7).
At first level, foes are doing 9 average damage against PCs with (assuming CON 10) 20 to 27 hit points. It takes 2 to 3 solid hits to take down a PC.
At 15th level, foes are doing 23 average damage against PCs with (assuming CON 11) 77 to 126 hit points. It takes 3+ to 5+ solid hits to take down a PC.
At 30th level, foes are doing 38 average damage against PCs with (assuming CON 12) 138 to 232 hit points. It takes 4 to 6 solid hits to take down a PC.
Higher level leaders heal a higher percentage of hit points per heal and have more heals, and PCs have more Daily powers (normal or items), so many players are more willing to use up a Daily every encounter or two.
Alternatively, a higher level party without several Strikers is at a disadvantage because monster hit points typically raise faster than PC damage for most PC classes. So, encounters are often longer which gives the monsters more of a chance to get that additional successful hit per PC. Players have to overcome this with locking down foes, focus fire, minimizing NPC mobility, etc.
But all in all, 4E is pretty darn easy as long as the encounters are N+1 or lower, or if more challenging, if the PCs are higher level.