At first I want to say that I very much like all features of the IDE and I feel very comfortable in it, writing Java code but there are two major things that make me not to use IDEA as my development envoirnment:
1) The IDE looks like a toy for kids....all those color icons make a professional coder laugh and ignore all the mighty features of this ide. I think it needs a new, more clean L&F design as a high-tech tool corresponding to the ease and useful thing in it. Everyone glances with admiration at the latest Mercedes and laughs at the Jolly VW Bug with pink circles:)) In Bulgaria we have a saying:"The lunatic at gay-colors is delighted":))However it's not so important - may be your product is not targeted at the hard coders used to write in Emacs.
2) In my work I use JNI a lot. The editting of C source code in IDEA is equal to edit it with Notepad. The syntax coloring is easy to set but there are no more facilities like these in the Eclipse platform for example - outline functions/members browsing and so on... It will be nice to browse the C source in IDEA as Java does.