Hardly a behemoth since I can run more tabs on Opera at the same time than on any other browser. Granted, I have quite an old computer but it's perfect for spotting differences between browsers. For others this is probably not an issue.

I use the mail client for my GMail and RSS feeds. Try it and tell me if you notice anything wrong with it. Actually I've been using the integrated client for so long that my perception about stand-alone mail clients is that they're a thing of the past. I guess it's just a matter of perspective, but for me it's far more convenient to open my browser and not think about checking my mail (it lets me know if new mail has arrived).

I don't use Opera Unite and nobody is forcing me to. It just stays out of my way, and so do the other features. Keep only what you like; just because they're there if you need them doesn't mean they slow you down.