Right now, the site (islam.com) is not something I would want to frequent. Whereas the StackExchange proposal process attempts to build a strong expert community before the site itself goes live, islam.com seems to have skipped that step and went live as soon as they could. As a result, it just feels like an islamic Yahoo! Answers with a StackExchange mechanic.
This may change in the future, but right now I can't see experts flocking to any site with a weak community and poor quality control. A site like this would need momentum to really take off, and they've barely started rolling downhill.
As Robert said in the StackExchange blog:
Area 51 is designed to build up momentum for a site prior to launch. On opening day, hundreds (soon to be thousands) of people pile in to ask and answer questions when the site first opens. So when someone asks a question, it gets answered… quickly.
They might be worth keeping an eye on as our proposal gets closer to public beta, and if they have built up a decent community we can work on how to either merge or to peacefully co-exist. But as of now, there's still a very open question of if they'll even be around (in the same form) if and when we do go live.
I tried the site out for about a week, and I stand by my assessment; experts are unlikely to flock to the site as it currently lies. It is barely moderated and most active users can't seem to tell the difference between a StackExchange-like Q&A site and a typical internet forum; voting is infrequent, comments are frequently posted as new answers, and downvoting anything is discouraged. Nothing like the type of expert-oriented Q&A site I'm expecting from this proposal.
It takes a lot more than superficial similarities in software to make a high quality Q&A site.