So I'm going to argue a few different points here.
First, it's not hard to cheese 4e. It's harder than it was to do in 3.5, but depending on what sources your DM allows, it's really not that challenging (especially if you use Dragon Magazine).
As I am sure you are aware, there is an entire CharOp forum, and when the game first came out there were people 1-rounding Orcus. Wizards has been wielding the nerf bat ruthlessly. Take a quick glance through the errata (sorry, "rules updates") and see how many things got better, as opposed to worse:
http://www.wizards.com/dnd/files/UpdateMay2010.pdf
So, empirically, building a broken character is not only possible, it happens with regularity.
Secondly, one of my main beefs with 4E (which I play on a bi-weekly basis and have occasionally DM'd) is that you basically need to optimize. The game is *highly* mechanical.
This is a continued by-product of a d20-based system, where a +1 bonus makes you 5% more effective (i.e. 5% less likely to die) than the guy standing next to you.
Even in trivial fights, optimizing for accuracy and damage is desirable because mobs (by and large) have huge numbers of hit points and do mediocre damage. So it's a long and drawn-out grind, and when you are repeating things over and over those averages start to really matter.
As a side note, I am disappointed by how wimpy monsters are in 4E. A level 18ish adult dragon went full attack on my barbarian for two rounds + an action point and barely scratched him. Fearful presence to stun, then two standard actions of attacks and an action point. Looking at the compendium, the max damage (w/o crits) would have been: 14+14+14+14+29 (claw/claw/claw/claw/breath) = 85 total points, 86 if the breath weapon recharged.
My character was level 10 at the time and had 90ish hitpoints. A *solo dragon* that was *eight* levels above me had zero chance of killing me in two rounds plus a standard action doing *max damage* using it's best attacks with *no reactions* allowed by my character. Pathetic.
I believe they are correcting this in MM3 by raising monster damage and lowering monster hitpoints.