So the Jarl said he would make me Thane, and he allowed me to buy property. The only thing left for me to do to become Thane is buy the house. The steward keeps telling me that some nastiness happened recently and the house isn't ready yet.

Anyone know why I can't buy this place?

I'm over level 20, and I have completed part of the Stormcloak storyline.

Well you have at least heard about the nastiness so that's a start. You must leave and re enter the city (fast travel out but walk back in from stables) a total of at least 4 times, upon the 4th time make sure you are re entering at night. If you go to the graveyard there will be a crime scene and a guard whom you can talk to in order to start "Blood on the Ice" quest. I highly suggest reading a thorough online walkthrough of it so you can see and hopefully avoid any bugs which other people have encountered when trying to obtain this house. Its a wonderful house to have and the quest line, when it doesn't mess up, is an enjoyable mystery. If you do happen to run into any of the bugs along the way, there is ways of forcing the house and the questline completion, but only if you can't avoid it.

Thanks, I have tried looking for that quest after looking it up on the wiki... so far no luck. It's supposed to be right outside of the Hall of the Dead at night, right? I'll try a couple more times.

I keep zoning in and out at night and checking...nothing
Maybe I missed the commotion once a long time ago and now I can't get it back?

You will notice the commotion outside the hall of the dead. The scene will have a guard (whom once you talk to will start the quest) and 2 or 3 bystanders. The key difference for my buddy who did this quest was zoning out, then making sure to walk back into to town after. He did a fast travel from markarth to whiterun, then would fast travel back to stables outside of markarth and walk back in at night so he would definitely know it was 4 full days and that he was leaving the cell and walking back into to town.

I, on the other hand think I went through the main quest too fast, had changed too many variables and the relationships I had with NPC's that I bugged it out. The only problem my buddy had was actual final completion status but everything else went fairly well.

One of the best houses in the game and it definitely makes you work for it. If worse comes to worse and you HAVE to have that house it is able to be forced through console but the work to make it normal (like if you had done the quest completely) is just as long as the quest with twice as much work. I hope everyone else has better luck with it then I did myself. Good luck.