If you are thinking about onsite optimization for a Drupal web site, there’s one module you really must install, SEO Checklist. Although it doesn’t do very much in itself (it really is just a checklist, with some added smarts in that it does check which other modules you have installed against its list of recommendations), it provides an essential baseline.
My experience has been that those who commission and build Drupal sites are not always focussed on SEO, so the science is not quite as advanced as it is with WordPress, for example. There’s a prejudice in some parts of the industry against SEO which I suspect may also be partly responsible for the slight lag in this regard. Still, articles such as the one I mentioned above may help to shift the balance as SEO practitioners become more aware of the many fantastic possibilities that Drupal can offer.