Here's a list of features I think would be useful for such a program.
Variety of chat types. You could have public group, private group, and private one on one chats and all would see use, I'm sure.
Chat moderation tools. Make it so the creator of the chat room is its owner and has control over various options (setting a password, muting/kicking/banning people, etc.), and allow them to set other people as moderators for the room as well, with varying permission levels. One of these chat options could allow them to set a delay like Magic Magnum suggested, or they could stick with the hectic mess of unlimited chat, since some people like that about real-time roleplaying.
Font customization options. People like to customize how their stuff looks, so give them different fonts, colors, sizes, and bold/italic/underlined options at a minimum.
User profiles. You should have something like what most chat programs have, where people can put whatever personal info they wanna share and an area in there for people to list their roleplaying interests and preferences. This ought to be accessible both by clicking on a user's name and by searching for them through some function of the program.
Character profiles. If it's for RP, you're gonna want to give people an area to describe their characters. The awesome ideal way to do it would be so users write up whatever character profiles that anyone can look at by going to their user profile, and when people are using the chat program they can choose one of their character profiles to use at that time; ideally this would change their displayed name to something designated in the character profile (say a 'Display Name' line under the character name line) and clicking on their display name ought to direct other users to their character profile page. Could be fine as just a section of the user profile instead of the fancy stuff with the name change and all, but I'm sure people would enjoy the extra touch added by the character profile swapping.
Friends list. If the program draws a lot of use rather than just a few people from this site, it'd be nice to allow people to have a friends list so they can see whenever people they enjoyed roleplaying with come online, plus have a way to open a chat with them with the press of a button.