What I would be looking for in a transfer, is a school and teacher willing to work with the child and the class, to make the transfer as smooth as possible. That's not 'it' though, surely?
Generally, transitions to public schools, when they are anticipated, are not problematical. The most common transition is from a class eight Waldorf school to a more traditional high school, and, from all reports, usually takes place without significant difficulties.
Transitions in the lower grades, particularly between the first and fourth grades, can potentially be more of a problem, because of the significant differences in the pacing of the various curriculums. A second grader from a traditional school will be further ahead in reading in comparison with a Waldorf-schooled second grader; however, the Waldorf-schooled child will be ahead in arithmetic.
Over our primary years we have had quite a few families come and go - moves interstate, moved too far away in terms of local travel, gone overseas, children arriving to us barely speaking English... staying for a while then returning back home (overseas) - you name it. None of the children that I know, have had a problem changing from Steiner primary to mainstream primary. In fact I'd go as far to say that they're possibly likely to be more adaptable in some ways, because of how they've been learning so far.
One Dad summed it up to me and said of his son who had transferred to a mainstream primary, that even though on day one, his son wasn't at the same place as his peers in the class in terms of some parts of the curriculum, by the end of the term he was at or above their level. Because it wasn't just the 'guts' of what he learnt at Steiner school that he took with him, it was also the way he learnt and how he approached learning, that he took with him.
We often have 'Parent Nights' where primary parents get to ask students who have completed primary at Steiner and moved on to mainstream highscool, how they found that transition, and would they have changed it to not have come to Steiner for primary, rather to mainstream instead. So far all of them have said the transition was simply a transition from primary to highschool (which is a transition for every child full stop) and that they wouldn't swap their Steiner primary years for anything else. In other words, it hasn't been unduly problematic just because of their Steiner primary education.
I've also listened/read anecdotes from university lecturers who comment that they can see the difference in the students that come to them from a Steiner background - and it's a positive difference. Being taught to think critically, use your imagination and think for yourself, be self motivated and eager to learn... all attributes any one could possess I guess, but they seem to see them regularly and especially so in students with a Steiner background.
So that's my thoughts. It's what I've seen personally, people I know, as well as what I've read and information that's been passed on. I hope it helps. Again, ask the your Steiner school about these things. They may have resources they can refer you to (I vaguely remember something about a study somewhere but I'd have to follow that up), or offer a Parent Night on that exact subject. Ask questions, absolutely.