Until the National Exam was over, I could have enough time to deal with the upgrade of WordPress. It took me some time to backup the database first, and then my customized theme and plugins. The whole process was smooth. It didn't break my data. XD But I don't like its new behavior about the automatically adding of paragraph tags.
In 2.0.x, it automatically does only when there's no tag before the first word of the paragraph. That made me can easily use <div>, <span>, etc. It doesn't violate the W3C standards when there's no <p> in <div> or <span>. This new behavior messed up some layouts, such as the Photos page.
Previously, I'd like to disable the feature, but this must screw up all posts. And I didn't want to spend my time in hacking the source code, so I used CSS to hide the effect of the newly-added <p>. It's not a very good resolution but acceptable.
When the softwares upgrade and upgrade and you don't want loss of follow-up, all you have to do is catch them and overcome the fear of changing from the old habits.
p.s. BTW, one of my customized plugin doesn't work now, which means I have to spend more time in it. T_T
2007.02.04 UPDATE: I replaced the wpautop function of 2.1 with that of 2.0.7. It seemed to work well. :-)