The 'Smooth Operator' (no alarm) and 'Ghost' (not seen by any enemy) add up pretty quickly. I think it's fairly balanced overall.
Taking someone down lethally nets you 10 xp. Taking two enemies down at the same time (reflex) with an unseen, silent, unarmed takedown nets you 72.5 xp apiece. The Smooth Operator and Ghost bonuses get added on top of that.
All in all, going in guns blazing and destroying obstructions gives you far less xp than going around everything unseen, which gives you far less xp then hacking everything and taking all enemies out (unseen!) with an unarmed take down.
Compared to the serious underperformance of guns and that you cannot be a total tank like in DX1, definitely shows that you have to play stealthy, hack all and take all enemies down as the pushed playstyle. You'll gimp yourself and make things much more difficult by going the "kill them all" route.
EDIT: And I expect you get rewarded for the unarmed takedowns as well because it allows the devs to show off their immersion breaking third-person animations.
Deus Ex 1 was all about choice. Play as you like. It all works, and it all gives you the same amount of xp. Deus Ex: Human Revolution is in many ways more like Splinter Cell and Thief than Deus Ex.
The devs of DX3 made it clear they don't like the RPG aspect, and want to reward player skills first and foremost: no weapon skills, sniper rifles suck (the good and silent one is only available through a DLC with some pre-orders!), and the harder the method is you use to take an enemy down, the more xp you'll get.
Boss fights. That says it all.
In DX1, I generally kill Anna by planting a LAM on the plane, and I either ignore all the other augmented agents ("bosses"), throw them a grenade or shoot them in the head with my sniper rifle. Once.
And while I do like the stealth approach best, DX1 doesn't register a difference between a lethal or non-lethal take down. That's why Paul and the armory sergeant both react like you killed everyone, even if you took all of them down non-lethal.
And it does feel great to go all-out bad-ass occasionally, be an unkillable tank and simply annihilate everyone. Which is something you cannot do in DX3.
Well, you might be able to annihilate them all, but you won't ever feel like that unkillable tank that takes down every enemy with a single shot.
Still, nice game, I like it, but I do have a long list of things I would like to see modded.