The swordmage marks an enemy. The Swordmage gets trapped by a trap. The marked enemy attacks who at that point? If the enemy attacks the already trapped swordmage, they get little benefit. If they attack someone else, the swordmage teleports out of the trap and gets a free attack.
Well, honestly, that's what the swordmage
does. And yes, their insane Aegis marking can get very, very frustrating. Really, though, it's all they get - their attacks are far from overwhelming. In fact, they usually do less damage than any other class in the game. They're highly-mobile, low-damage defenders. And traps like that just won't stop 'em for long.
Remember it has a range of 10, though, so if the fight somehow gets 10 away from the Swordmage, that aegis won't do a thing.
(And if you're frustrated with Aegis of Assault, you should be thankful your resident swordmage didn't pick up Aegis of Shielding. It's a hugely effective way to completely hose a creature - especially with swordmages' ultra-high AC.)
In general, I struggle with marks and I imagine the new avenger doohickey will be the same. If you are a monster, what do you do? The PCs really seem to have the opportunity to dictate the fight more than the monsters do, as few of them can mark. Few of them have the synergies available to them that a well rounded party does. As someone that used to be pretty good at DDM (the tactical game), I should be able to run the monsters so they are more than just a lot of hitpoints to slog through, but I often feel the monsters' choices are very constrained.
Their choices are certainly constrained if they are stuck working in a vacuum or a bare stone room. Interesting environments are almost a must for 4e games... It doesn't take much effort to throw a few braziers, a little difficult terrain, and maybe a chasm or two on the battlefield.
You can do some great stuff by looking for creatures that work well together. Hobgoblins are probably the best example of this that I can think of - they tend to get insane when you put enough of them together. The Archers get a ping-ponging combat bonus, the Soldiers get huge AC boosts, and everyone moves well in formation.
Have you checked out Pyramid of Shadows? It's got a lot of interesting terrain and encounters. I'll note that it's frustrating since most of the terrain only hoses the PCs, but still it's a good starting point.
-O