Their spelling model isn't as useful or complete as Google's. [zasie san francisco] ought to suggest [zazie san francisco]; at Google it does, at bing it does not.
The recent searches tool is distracting because it's in the same part of the screen as suggested searches that other people have done. I searched for the (misspelled) [gureye cheese] and a similar search in the left bar caught my eye: it was my own, and it was spelled incorrectly. I'm not exactly sure what information from my search history would be more useful there, and I know that smart people from both Microsoft and Google are trying to answer than question. Keep working on it, folks.
bing's typography is harder to read than Google's. The text on bing feels thinner (may be an illusion); certainly, bing chooses to highlight fewer words in the search results (eg, it does not seem to highlight the terms in result titles or URLs). Highlighted words let searches skim the results more quickly, which generally is what I want to do because I expect half the results (from bing or Google) to not be relevant to my search (because I, knowingly, provided less information in my search than what I need for my task).
I've never used result previews to any benefit; if the snippet looks promising I go to the page (in a separate tab); the longer text-only snippets don't feel like they provide any more detail. And, in fact, they're distracting to me, because I seem to frequently trigger them by hovering my mouse over some hot spot on the page. As a result I now to longer know where I can safely leave my mouse on a search results page.
I miss SearchWiki (of course, I ought to). In part, I miss being able to mark a result as "not what I want" or "bad in general". In part I miss being able to bookmark a page I encounter in a context to be recalled when I search. Eg, I have a SearchWiki for [lampworking tutorials] to which I add individual tutorials and libraries of pages as I encounter them. Anytime I search for [lampworking tutorials] I want to see my results (or a digest of them). I'm waiting for the day that U Rank appears on bing.
I plan to leave bing as my search engine of choice for a few more weeks. I hope these observations will be useful at some point in the future. Please feel free to comment with your own bing vs. google usage reports.