There is no feature, compelling or otherwise, that Bitcoin can't replicate. The only question is whether or not the userbase considers your 'killer feature' as highly as you do. If they do, then Bitcoin will steal your idea. I'm still waiting for your grand announcement, BTW, and I'll be the first to point out if there is prior art within the bitcoin ecosystem.
Bitcoin has certain aspects it cannot change without disrupting the system to the point that it could collapse, which means it is vulnerable to competition. For instance it cannot change its rate of coin creation and would not be able to change its hashing function unless it was near collapse or thought the risk of collapse was worth it. Bitcoin will be overrun by a crypto-currency that is better in some core area, because by the time Bitcoin will make the change it will already be near collapse, and the emergent currency will have all the momentum.
Furthermore, just because it can be changed doesn't make it invincible, there are limits to the speed and scope of the changes that are possible. If the ability to adapt makes a currency impervious then we will be stuck with dollars forever as the FED and U.S. gov't control what the dollar is and has changed this from 100% gold backed to what we use now, fiat.
Finally it is highly unlikely that it will be clear to the masses at an early stage which currency is superior, and the only scenario that would allow Bitcoin to adopt core changes with speed would be if all the users understood that the newer version was superior. But who understands issues at Bitcoin's and more generally money's core? Almost no one so Bitcoin has vulnerabilities.