Besides things such as the language attributes in script links, there are some further parts of HTML you may be used to using that are now considered "obsolete" in HTML5. It's important to be aware that there are two camps of obsolete features in HTML5—conforming and non-conforming. Conforming features will still work but will generate warnings in validators. Realistically, avoid them if you can but they aren't going to make the sky fall down if you do use them. Non-conforming features might still render in certain browsers but if you use them, you are considered very, very naughty and you might not get a treat at the weekend!

In terms of obsolete and non-conforming features, there is quite a raft. I'll confess that many I have never used (some I've never even seen!). It's possible you may experience a similar reaction. However, if you're curious, you can find the full list of obsolete and non-conforming features at http://www.w3.org/TR/html5/obsolete.html. Notable...