I don't think interns need any particular technical skills, but rather a positive set of values and characteristics to apply in any given situation. It is imperative to have a positive and ambitious mindset to always grow and learn more about the firm, industry, colleagues, clients, and your professional self. This entails not being afraid to ask lots of questions to better understand the big picture, and requesting feedback from your engagement team on what you are doing well, and how you can still improve. Basically, demonstrate that you genuinely care and love what you are doing. That mindset will make the entire experience that much more fun too.
I think leadership and teamwork are important skills. In addition, being a quick learner, being detail-oriented, and having an analytical mind are critical. There are a lot of programs we used in PCS Tax, and my ability to quickly catch-on was immensely helpful, not just for me, but for them. Paying attention to detail is also very important. There are many minute aspects of a tax return, and being alert to those details helps the whole process move more efficiently. And having an analytical mind as you are looking at the provided tax information and reviewing tax returns for accuracy are very important. Lastly, I'd say being personable is a great asset. Being able to communicate and work well with others makes a more enjoyable work environment and are things they will be looking for.
Interns are very fortunate in that they aren't expected to know much coming in. As long as you have taken tax classes and have a basic foundational knowledge of tax concepts, you will do great. Interns will benefit greatly from developing networking skills and general people skills, and a majority of your success will be determined by your ability to listen, follow instruction, and work well in a group.