You can't blame projects for not immediately supporting your specific browser. Most teams develop for the standards. If your browser doesn't conform to the standards that work for major browsers, then it's just harder on the developers.
What is unfortunately happening is rather than testing to see if the site will work in Opera, developers just exclude it. They've made lots of progress over the past few years to get their browser up to snuff. I just recently switched to Opera after Chrome started lagging during the 1.6 update.
Idealy, developers could target one set of standards and their product would be functional across all browsers that implement them. In general, the bigger browsers all act relatively the same (not IE) when standards-compliant code is used, so if what works for them doesn't in Opera, then that's a problem.
It's not the developers' jobs to make their sites work with Opera (especially when it works in IE, Chrome, Firefox, and Safari). It is Opera's job to make sure they render sites comparably to other leading browsers.
If that means they rip out their rendering engine and replace it with Webkit, so be it.
Coding wrong and passing the problem to the browser is just mean
BTW, different rendering engines, assure no monopoly, innovation and competence, which is always good.
Go through them and let me know if you still believe that Opera doesn't try hard enough. The problem is that being standards complaint is harder than it soudns. A lot of the nitty gritties aren't explained in standards and diff browsers implement them in diff ways. While developers explicitely implement patches for Fx and IE, they generally ignore Opera. And that's why so many websites don't work in Opera.
And that's wrong, css attributes whose drafts aren't final yet should not be labeled as such by Opera.