I love this lunge rule!
Sure, it probably nerfs controller powers a bit, and benefits enemies more than PCs, since a lot of enemies just use their melee basics to attack anyway...BUT it is a good rule that should be incorporated. If you want to be truly safe from enemies, you need to be far, far away from them. Or hiding behind obstacles.
However the idea that some hulking brute of a dude in plate armor can do a move, melee basic, and another move as a standard Lunge action is waaaay broken, with or without an extra OA for the target. That's a power more in-line with a rogue's shifty power (the shift is still useful and more beneficial than a straight-up 1 square move, against enemies with threatening reach or when the battlefield is crowded). Being able to both approach and then leave every round is too beneficial for too many builds. Being able to move in, strike, and move out, is typical of ranger or rogue dailies, not some at-will you give to every class.
I would definitely make Lunge a simple, 1 square move then melee basic, that's it that's all. Your turn ends next to your target, after the attack. (or not, why penalize those of us with the foresight of picking minor action attacks? Or, lunging, and if you kill the monster, move over to flank with your ally over there. Why not? I like not ending your turn for no good reason. Chances are, you already have your move action wasted anyway, but if you don't, use it after the hit!). This makes is sort of a "back-loaded" anti-charge. A ranger could get up from prone, Lunge, then minor action attack. Or someone else could shift back, then Lunge some other enemy in a more advantageous flanking position. There are plenty of fun combos that aren't overpowered with this.
This is the rule that's been missing from 4e that makes it a much better game, IMO.
If we are to talk about adding an OA for the enemy because you are opening yourself up in a risky move, you would HAVE to apply the same reasoning to a charge. Sure, you get better to-hit and movement and all the side-benefits of charging items and feats and so on, but THAT should grant CA to the target (if you hit) or provoke an OA (let's say if you miss).
Or maybe do it as a house rule with a dice roll mechanic, e.g. roll a dex check vs DC 20 for an OA, or else vs DC 15 for granting CA to that creature. These types of things would make charge-maniacs pay a price for too much cheesy charge optimization. Running up to an enemy to strike it is a bold, risky move.
You should have to trade off some penalty, granting combat advantage every time is probably best + simplest. I.e. never, ever charge a rogue on the open battlefield. (giving him CA where he would have a hard time getting it on that turn otherwise).
One more anecdote. I was playing this ranger one time and had this shifting power as an interrupt. Sure, it's a good power, but only because of this 1-squre donut hole. It moved up to me to do its at-will, (not charging), when it stopped next to me and before it could hit me, I shifted back ONE square, denying him the chance to hit me at all. What did he do? Just charge the wizard. Tactical fail on my part, and fail for the rules. If I had forced the guy to lunge me with this power in order to not have his turn wasted, I believe being given CA to me would be just compensation. The power could still deny actions if you shift two+ squares away behind a barrier, wall, or column, where the charge could be prevented.