I used to an be external contributor to open-source GWT, and then became a Google employee to work on the GWT team and the situation really has nothing to do with elitism or culture.
It basically boils down to a matter of resources. When I started as an open source contributor to GWT, it was used by external developers, but not really used internally by Google, so changes made by external committers couldn't possibly break anything.
Slowly over time, more and more Google properties started using GWT, and suddenly, you had the situation where an external user could submit a patch, that passed all the GWT unit tests, but broke major Google properties (e.g. AdWords, Google Groups, Wallet/Checkout, etc) Google builds everything from head, so when you do an internal commit, not only do your unit tests run, but the unit tests from every project that depends on GWT, so you find very quickly if your patch broke real applications. This happens all the time. I commit, pre-submit queue for GWT is green (all tests past), then hundreds of other projects get their chance, and there's always 1 or 2 that break, not always because of GWT per se, sometimes because of bad code in those projects.
The problem is, there is no way for external committers to get notified of internal (potentially confidential) apps breaking on their changes. This meant that every external commit would need to be reviewed and proxied by someone on the GWT team.
Now, back when we had over 20+ people working on GWT, it wasn't hard. Now there are only 5 full time committers, and it has become a lot more difficult to keep up with the external community and support internal users.
I have been internally advocating that we "re-open source GWT". That is, we make the "source of truth" be an external repository, possibly re-hosting it on GitHub or Google Code, and fork it off from the internal version. We grant all our of best and dedicated contributors rights to administer and commit on equal footing with Google employees, and we run external continuous build systems for it.
On the innovation front, I think it's true for gwt-user, but for the compiler, I've hardly gotten any external contributions for optimizations and almost all of the improvements in speed and code size have arisen internally. This may indicate that we should run separate open source projects for the compiler/tools and the libraries, splitting them up and separately managing them.
But the GWT team IMHO was never elitist, just saddened that contributions were piling up and we lacked the bandwidth to review and commit them in a timely manner. I feel bad about it, given the time people put in, and I've been spending time recently trying to collect all outstanding patches for landing into GWT 2.5.