main reason is the activity here... you post something, you get a reply very soon. that's why people come back. without interaction, there is no fun. but the question is how did the activity come in the first place... probably due to some authoritative and influential members - people come here to seek help and advice.
So may people from around the world works and earn their living here at DP. There's so much valuable informations around and by using them + your imagination produce new creations. Now I wonder how big DP would be in the next 5 years.
This is the first time I've visited the homepage. It looks like it could use some work . I think part of DP's success is the knowledge of its users combined with rather strict moderation of the forums. That's right, I'll give credit to the mods who clean this place of spam and other useless posters. There are very few forums that are this strictly moderated, and frankly most of them are annoying because the spam:content ratio is so high.
i know several forums which have way more information than dp, and there i spend most of my time, but i came here cause there are plenty of noobs and there are plenty of easy money floating around, and you can outsource work very easy and very cheap.