This is the question I've have been thinking of lately as well.
My site, http://www.kravly.com
, is new and everytime I send it out to my friends and family, many of them would choose to view it with their mobile phones. I always to have then shoot them back a message saying that I didn't optimize it yet for mobile.
Fortunately, my site already has a responsive template code in it, so it automatically is formatted for mobile usage, but the problem is is that I haven't optimized it yet.
Things that are formatted for the desktop version, don't necessarily look right in the mobile responsive version. For example, you may have a top menu with many items, but when viewed on a responsive layout, the menu becomes a drop-down that may contain too many items. Thereby cluttering up the usage and not ideal for mobile.
I think what also needs to be factored in when creating a mobile/responsive version of your site is that you need to think about what type of content users are looking for when surfing a mobile version. (Thiis has been said in previous posts before as well.)
For example, how I use my mobile phone for surfing is primarily to look up information that I need right away. A lot of times, its things like, store locater, prices, product details. I rarely ever take too much time to consume huge amounts of information, and I hate it when I need to enter any information at all because well, a tiny 4-5 inch screen is not optimal for entering information.
So, if you can make your mobile site cater to those needs, I think it would be ideal.
Now that you have asked this question, I think that has to be a new priority of my things to do list as well, since I haven't optimized it for this.
My site consists of users who just browse and consume info and then there are users to create content which contains a lot of info, which I don't think is the ideal use case for mobile users, they'll just wait until they are at a desktop/laptop for entering lots of info. So I should cater more the those users who just browse, or want to create small items with minimal info. (But still provide the option for long usage if possible, since some users don't mind.)
Anyways, I hope this helps. My first post by the way!