Mike,
I'm of those who had alway used Pleco for my flashcards and then when the iPhone version was released migrated over to Anki in order to continue to be able use flashcards on my phone. Once I made the switch I realized it would be extremely difficult to ever go back, although whenever your release a new version I always go and check out the flashcards.
Here are some things that I've come to love about Anki that I'd like to see implemented in Pleco.
#1 & most importantly: The ability to add media- I have a lot of sentences that utilize recordings of natives speaking an entire sentence. Being able to use SRS for listening practice was a revelation for me and is now a large part of my flashcard deck. Also, part of my deck consists of food ingredients and dishes, without pictures in my deck these would become almost useless as learning the name of a dish isn't very helpful without knowing what it is.
#2 & almost just as important: Statistics- The more the better. Keeping track of my progress is extremely important. Someone already mentioned the HanziStats plugin and it's a feature that I love. Although, even more importantly is keeping track of review history. I know it may seem silly, but being able to see a graph with my review count and time keeps me motivated as I know if I don't do my reviews I'll always see that dip to zero on my history graph.
#3- Ability to easily add sentences - I love being able to add sentences to my deck right on my phone when I encounter them.
#4- Faster scoring of cards - In Anki, there is a default answer, based on your history with that particular fact, that guesses how well you know the fact. I have found this to be spot on almost all of the time except for when I have forgotten a fact. In Anki I have it set up so that different area of the screen correspond to different scorings, so I can set up tap zones for default and incorrect which allow me to fly through my reviews. The current system that you have looks much prettier, but I have found it takes longer and even small differences in time add up when you are doing hundreds of reviews a day.
#5- Syncing- A nice feature, but I'm not sure it's as important as you think. Most people I know that use Anki only review on their phone. So while it is really nice to add cards from the computer and sync them, I don't think that AnkiOnline is the main reason people use Anki.
#6- Accept Custom Fonts - I love being able to use the KaiTi font in my flashcards.
#7- Edit cards on the phone - With the dictionaries built into Pleco, I wouldn't have to worry about incorrect definitions for words, but for sentences this is important as well as for adding notes to cards (especially important for adding mnemonic devices).
And one last feature specifically for SRS and one that isn't included in Anki, but I'd love to see...
#8- Local notifications when a set number of cards are due to be reviewed
I love Pleco and find it indispensable, but for me these things would have to be included in order to convince me to switch back. For now, I use Anki for SRS and then when I need clarification on a definition I copy the words or sentences and switch to Pleco (via an Activator swipe) which looks up the word or the sentence in the pasteboard. I have found this to be the best of both worlds as it essentially integrates the two apps.