Generally speaking for power user every activity on iPad is strictly worse. I can't easily download ZIP, unzip it, open some text file, edit it, send it via Mail. Probably I can do it with right apps, but it would require much more clicks or taps.
What I can't even imagine doing on iPad: using Intellij Idea, using XCode, using Google Chrome to debug and develop web apps, using image editors like Sketch and Pixelmator (I know that I can get some kind of image editing, but I don't think that I can do what I'm doing on PC).
Now things I could theoretically do but probably can't, because of walled garden: using Terminal to embrace full Unix power, downloading files with BitTorrent, using BitCoin. Probably possible with Jailbreaking, I'm not sure. Also I'm not sure whether I could download some huge 20GB file and watch it using another app without duplicating (does iOS copy file when I open it with other app or just hardlink?).
And, of course, keyboard is necessary. Mouse would be useful too, but iPad doesn't support mouse, AFAIK.
So probably the only users who can easily migrate from PC to iPad are very casual users, who use their devices to browse web, chat and play simple games. There could be some professionals who work with iPads, it's theoretically possible, but I can't imagine anyone.