It places the IP address inside an encrypted packet. At best, this means that the internal address must become public - negating NAT's security benefits. At worst, it places that IP address inside an encrypted packet, leading to routing problems. Certificates, too, remain a sticky issue for user authentication. So, of course, vendors are filling in the blanks themselves.
Interoperability tends to get derailed by other vendors.
That's hard to say because there are no objective sources of information, other than the trade press. Vendors are basing their drafts on technologies that give them the fastest time to market, not necessarily the best standard. Of course, Microsoft, too, has implemented IPSec over L2TP in Windows 2000. The best source of information would be other users, but they aren't involved enough. Although the IETF is pretty open, users don't get much of a voice.