Well, it is certainly a matter of opinion whether or not it is still 'easy' to play with a classic 360 D-Pad. However, from a strictly design/implementation standpoint, it is inferior to a classic D-Pad, in that it requires more precision in the button press.
The 360 D-Pad is a solid disc, as opposed to separate buttons or a strict shape cross shape. What this means is that for the pressure points for the u/d/l/r inputs under the disc, if you want to input a 'diagonal', you would just press a different part of the disc, as opposed to two different buttons at once. The question becomes, at what point is the pressure on the disc going to affect more than one input? One article ('study'?) I read years ago on the subject found that, at the minimum force to register an input, only an estimated 12-15 degree deviation in either direction will hit a second input. In other words, 2/3 of the D-pad disc registers a diagonal, meaning it's 'harder' to hit a single direction than a diagonal. More force on the disc results in a smaller deviation required to register the second input, which explains why it's tougher to play fighter games or action platformers like Super Metroid in the heat of the moment! Add to that my own personal theory that the disc shape itself allows for inadvertent slight rolling of the pad, and you've got a tough situation, and an unforgiving 'D'-pad.
Of course, you're welcome to call the article's exact findings bullshit (or maybe call me a bullshitter, since I can't find the article ), but the principle makes sense. When you have separate buttons for a D-Pad, or a stiff cross shape, accidentally hitting two directions at once becomes much less a problem.