I find that when discussing the difficulty or lack thereof of 4E, especially its published adventures or official encounter design advice, it's important to note that there's a very wide range of possible "power levels" for the PCs. This is true of most RPGs, of course, and was definitely true in (at least) 3.x D&D as well.
By "power level" of the PCs I mean two things, mainly. One is the skill of the players -- their experience with 4E, their experience with D&D or RPGs in general, their level of system mastery, their tactical-mindedness, their "powergamery", and so forth. The other is the degree to which the characters are mechanically optimized or well-built to take advantage of the rules, handle their roles well, and generally kick ass.
I've seen VERY different tables full of players as far as this goes.
There are players who TPK in every adventure out there, who complain about certain encounters being "way too hard", and so on. Newer players, inexperienced gamers, or people who make less combat-focused or mechanically strong characters can be quite challenged by published scenarios at any level, and I think that the core encounter/adventure design philosophy which is expressed in the DMGs and evidenced in the written modules is mostly aimed at this sort of PCs. Fairly mediocre players running fairly weak characters. If this is what your PC group consists of, then you'd do well to ignore the "it's way too easy" comments, because the official encounters are probably tuned just about right.
But there are also PCs who are considerably more potent, both because of strong character builds and experienced, tactical players. When you have PCs like this, the published encounters and official design advice really fail to challenge, and this becomes more and more obvious as the game goes into higher and higher levels. So a lot of the anecdotes you see here about how the modules or the big named enemies are so very easy are coming from groups with these above-the-curve PCs.
I DM for and play with some guys who are really quite brilliant at D&D, and have 4th edition in particular mastered. This includes myself. We build very strong characters, and we know the game very well. We can handle a LOT. As a DM for these guys, I have to step up the difficulty dramatically in order to challenge them at all. As a player, I nearly always find myself experiencing the game as extremely easy, and urging any DM I play with to make the encounters harder, harder, and even harder.
So, from the perspective of skilled 4E players with a tactical mindset, using well-built (for combat) characters, yeah . . . the monsters are really very easy. The published scenarios are really very easy. The special named NPCs who are supposed to be a really big deal are, in fact . . . really very easy. This becomes more true the higher level the PCs get. At epic tier, you have to really completely abandon the official encounter guidelines in a big, big way in order to legitimately threaten a party of "strong PCs".
So, in order to know whether or not Orcus is going to be scary and "epic" feeling to your PCs, or a joke that they'll trounce like a kid who owes them lunch money, you really need to be aware of how "good" your players, and their characters, are. But in my experience, if they're even "fairly decent", they'll overpower just about anything as written without too much risk.