I am not sure if there are any "real" competitors for DP. But I would say, DP is becoming a serious competitor for Sitepoint
I believe there aren't any, in terms of activity and number of users there is nothing to compare DP to. Other smaller forums do a better job at more specialised subjects though.
Sometimes i feel DP is almost too big in terms of number of users etc, that alot of useful information gets missed or poor information gets provided.
Compitetor : for domain section forums like dnforums for hosting related webhostingtalk for SEO,marketplace,developement DP is very big then others. over all for webmaster forums DP is number one with SP,webmaster talks,v7n behind it.I think if SP stop searching ways to earn max out of forum it may be considered as serious compitetor.lol i cant say much about them as admins of those forums are members here .Other day as usual slipped my tongue and made very clear post later came to know he was real admin of 1 of those forums.
I'd say SP is in the lead but DP is catching up very fast. But they both have their niches, there is plenty of room for both sites. SP is geared more towards coding, servers, and that type of stuff. DP is obviously geared towards SEO. SP's marketplace is also better than DP but it also costs money to sell stuff there. The only time I use SP is when I need to sell something. DP FTW!!!
I would say webmastertalk is certainly feeling the heat. If you post something containing the word "digitalpoint" there, it will be replaced by #!$#$@.