I rarely if ever pad, which you might not believe. In fact, I anti-pad. My writing is so skeletal in the first draft stage and lacking in description that I often have to go back and add these things. Of course, not every Wrimo writes like me. Padding can be part of the NaNoing experience, but only if that Wrimo chooses to make it so. The idea behind padding isn't necessarily to make one's word count or to intentionally write crap that will definitely be cut out later but to write something down. You can't edit a blank page, and writing anything down lets you keep going. In that way, NaNoWriMo fosters the best writing habit of all if you want to write well: writing.
Word count, not page count, is what the publishing world usually goes by. Anyone who has written a paper for a class knows that you can fudge page count by changing the font or margins to squeeze in that last half page. Word count can't be fudged as easily, but you get a good idea of how many pages, say, 50,000 words take up in pages. (Somewhere around 175 book pages, if you didn't already know.) Awareness of your word count isn't an entirely bad thing in the long run; it makes sure what you intended to be a novel doesn't end at ten thousand words or stretches on to 300,000. Some genres have understood guidelines on word counts, so knowing those limits going into a book's writing is a good thing so you don't overstep them too much. Well, unless you're absolutely convinced that your novel is really really good.
If you're a writer outside of NaNo, there's a very good chance you're going to work under a deadline at some point. NaNo teaches you how to do that. Writing under a deadline makes you better at it over time, and you'll learn how to write smoother prose that is easier to edit, even if it doesn't look easier at first. Take this from my own experience; my first NaNo novel is never getting touched again, while two of my books from the past two years definitely have potential.
If there is a bad habit that a Wrimo may fall into, it's the idea that one writes only in November with a bunch of other writers. Writing doesn't have to be a solitary activity, but it's not always an activity with thousands of writers cheering you on, either. Not everyone does NaNo with its breakneck writing speed and community support, and as much as I love NaNo, that's okay. November is for writing, but if you want to be a serious writer, the rest of the year can't be off-limits.