Pro Activly developped addon/plugin-ecosystem
See here for Anki 2.0 and here for Anki 2.1.
Addons can be rated. With older plugins those ratings usually reflect its overall usefulness quite well. Mostly you can actively participate in suggesting updates and features on the according github page.
Examples for good plugins that actually should be default: Night Mode, Zoom...
Pro Ability to create subdecks and filtered decks (and specific options for each deck)
Subdecks: You could, for example, split a field of knowledge in seperate parts that you can then study seperately.
Filtered: You can focus your review on failed cards, recently failed cards or anything else (really anything), just to name some examples.
Pro Highly sophisticated evaluation (statistics page)
Statistics page which can also be highly and highly individually be enhanced with useful information about the reviewing process. This way important adjustments of the reviewing process can rely on solid grounds.
For example, if you are content with a retention rate of 75% of your materials, it reflects in your statistical overview if your learning configuration is chosen right to achieve that goal (or if you're overachieving and thus spending too much time).
Pro Highly versatile manipulation of cards and notes in the browser
Even batch processing to a high degree of automation possible.
For example: You can enhance your card with machine spoken audios automatically (addon: AwesomeTTS). This enables you to do reviews (on AnkiDroid) in your pocket (using swipe gestures and headphones), which is convenient on commutes.