I...just can't believe people are calling that skill. Ventus was flailing around aimlessly out there...
I really don't think it's that Demise is an easy boss, as many people have tons of trouble with him, just that you're a skilled player. People say that Demise can be beating easily by using certain strategies, or by using easy patterns, but even some of the supposedly easy cheap strategies that always work can be difficult for me to pull off. You have a really good grasp and understanding of the WM+, and are just good with Zelda in general.
I'm not a skilled player (not trying to be modest here; what I'm saying is that I'm not a great player), and remember that I'm not even using WM+ proper - I'm using a crap emulation of it using keyboard keys. Even with keyboard keys, Demise was easily taken down by a sloppy player. Yes, I made a lot of mistakes, but that's the point: Demise is a forgiving battle even without a shield. It wouldn't be fair of me to post a 30sec run of Demise (e.g the video DarkestLink posted) and claim he's the easiest boss ever; THAT would be me being a skillful player.
About Demise attacking you twice, that is only due to your proximity to him. As I mentioned earlier, you can manipulate what he does just by being close to/far away from him. If you see him slash once, backflip and move away - otherwise he'll double slash (triple slash on second phase if you shield bash). He's not a sporadic type of boss.
Any boss that you can defeat within a short period of time is a boss with little depth at all.
How? That's like saying any long boss battle is automatically filled with depth. All I have to do to prove that statement wrong is point you to nearly ever GameCube Zelda boss. A boss can have plenty of depth and be capable of being defeated quickly (I point you to many of Metal Gear Solid 3's bosses). It just depends on the specific mechanics of the fight.
Any boss that you can master within a couple wait no 1 boss battle has little depth, in his first phase he had very few attacks and it was a simple pattern of block and strike, very repetitive, easy and no depth! The second phase was intentionally more challenging and he had more attacks but his concept of attack remained the same, he would repeate his routine and one could notice this straight away. I've had to do more figuring out how to defeat a boss with a Mini-Boss than Demise himself.
Depth doesn't come through time. Depth comes through options and methods of approach, and Demise has plenty of those.
Is depth of attacking used to it's full potential when Demise can be defeated within 1-2 minutes?
It depends on what kind of method you're using. Demise can take a long time or a short time. It just depends on what method you use and how good you are at using it. That's great depth. Plus, Demise is the third portion of a three battle boss sequence. It's not like he was the only final battle.