Not to honor 3e, that's for sure. To encourage PCs to use interesting terrain features. Right now, I can't get them "past the door" so to speak without putting an innocent in imminent danger or having an ambush. This was a beef (not the biggest, assuredly) I had with 3e and I was hoping 4e would have some motivations to help correct it. On the contrary, 4e's reliance on party members makes it even more likely for the PCs to just stop at the doorway (or equivalent). It's really frustrating to DM.
There are other incentives to moving around. Cover is a good one and a more sensible result in most case. If you have a fight with a lot of artillery monsters and there is an hard to reach area that offers good cover, it's a good gamble to take it.
A trap with an ongoing effect also creates a good incentive to reach the control panel.
An entrenched enemy that causes a lot of trouble may encourage the rogue to go get him.
etc.
It is easier to lower your guard to defend than it is to sustain your guard high against faster, harder blows.
No it's not. Try protecting your feet. See how much strenght you can put on a parry this low or how akward it is to lower you shield that low and in what position it puts you.
Meanwhile, all your high blow come at a nice, predictible angle.
And did you say rapier? Rapier is a god awful weapon to use when fighting on uneven terrain. I fenced for many years and it is absolutely critical to be able to move back and forth freely when fencing. The style of fighting and the weapon are always inseparable but in the rapier's case, it is critical. It is a very specialized weapon that becomes a useless lump of metal if you can't fight with it the way it was intended.
While parrying is a basic fencing skill, the best and most essential defense is simply backing away just the right amount (preferrably while also trying simulteneously to strike the arm of the attacker). When you fence, being a keen judge of distance is key and retreating just enough so that the attack can't touch you while you still can reach your opponent (either reach his entire body if your reach is superior or just his extended arm if it is not) is basically the most common way to get a point (and in a duel, a win). At my peak, I'd routinely back up just so that the opponent's attack ended litterrally one inch from me while my attack landed sqaurely on him.
And the most basic attack is simply a sudden lunge, often prefaced by a feint. Try to do that if you are fighting in a stairwell or on a table. Lunging becomes impossible and therefore your offense is crippled.
Due to a long history of swashbuckling movie where fighters favor longs exchange of short jab and parries and where jumping on table is seen as a good idea, I wouldn't actually penalize this behavior in D&D. But I sure wouldn't reward it either.