This is an interesting prospect.
First, I just want to clarify what I thought the OP meant. First, remove the 1/2 level bonus from the PC's stat calculations. Then, subtract 1/2 of the challenges level from the DC of any target number in a challenge (be it a skill challenge DC, monster defense, or whatever), and the attack rolls that the challenge makes against party defenses, regardless of whether 1/2 of the level was used to calculate it (in most cases, it is not).
This will produce NO CHANGE in hit or success chances against challenges of your party's level. You just subtracted the same number from both sides of the equation.
I see this having three (or four) major effects.
One. It will tend to shift existing (that is, written without this rule in mind) challenges toward a single difficulty. That is, previously easy fights will become harder (since the party lost more from the D20 than the monster/skill challenge lost from the DC), and previously hard fights will become easier (the opposite).
Two. It will make writing new creatures and challenges more difficult. Currently, if you write a new monster, you have some padding in the design process. If the powers you have chosen are a little too good or too bad, the fact that the creatures attacks and defenses vary so aggressively by virtue of the simple level math smooths it out. Taking that away, even only by half, makes the balancing act that much harder.
Three. There will be weird fringe effects. What about truly fixed DCs, like wall climbing and gap jumping? High level characters no longer improve at these. Unless every wall now has to have a level as well as a DC, in which case, the higher level you make the challenge of the wall, the EASIER it is to climb (since it starts at a fixed DC, and you subtract half the level). DCs set by tier (such as monster lore, trapfinding, lockpicking, and so on) will have a "sawtooth" style plot of success chance based on the level of the device/monster, where, for the same character, it will be EASIER to pick/find/learn about higher level challenges than lower level ones, for the same character.
My point is, while an interesting idea, it would be really really weird. Unless, of course, I misunderstood the original idea, and the proposal is to remove 1/2 level from the players side, and whatever level based math went into the monster side (mostly full level).
In that case, the game would quickly become completely silly, with PCs being completely and utterly unable to miss by mid Paragon tier, and monsters hard pressed to ever hit.
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gnfnrf