When I crossed the border to Kazakhstan I was told that I had to register my visa within 5 days. I read about that before, but I thought I had read somewhere that this wasn't necessary for EU citizens, so I was a bit surprised. But it turned out registration wasn't that difficult.
I did the registration on Monday in Astana. Registration needs to be done at the Migration Police office, which is located in the
улица Сакена Сейфулина 29. I was there at 11:00 and the place was quite crowded and busy, so I already feared this might take some time. But it only took a couple of minutes, I gave them my passport, my migration card and a business card of my hotel (they wanted to know the adress). Then I was told to come back at 15:00. I was also told that they could only do a registration for a maximum of ten days. This wasn't a problem for me, because I don't intend to stay longer in Kazakhstan and my Visa validity ends there anyway. But for travellers wanting to stay longer in Kazakhstan this might be a cause of trouble.
After all, it wasn't that troublesome, but you should be aware of the registration when travelling to Kazakhstan. I don't know what to do if you're not passing a big city and thus have no way to visit the office of the migration police.