On top of that, PowerBI is subsidized in order to funnel customers for SQL Server.
Even with open source tooling, it's a large and ongoing development effort to do BI right.
I'm not saying that to bash on PowerBI. I love that product and use it heavily and promote it to clients. It just serves different purposes than Tableau.
Out of the box mapping is miles better in Tableau.
Overall Tableau is more fully-featured than Power BI. Tableau aims to be a complete BI presentation layer. Power BI is positioned as the self-service and personalized consumption component as a part of a larger BI presentation layer. Microsoft would prefer that the entirety of the BI stack be made up of their technologies, but Power BI can consume other data sources, and its reports can be embedded in other apps, so it fits into other technologies as well.
We could go feature-by-feature and Tableau would win the majority of presentation sophistication bullets (more fine-grained control of display, filtering, interactions - richer collection of built-in visualizations).
The differentiator for Power BI is more on the self-service end. Personalized dashboards (dashboard and report are two distinct concepts in Power BI) can be trivially created from published reports. Customized reports can easily be extended from published reports and datasets. There's a strong collaboration framework based on Office 365 groups and with lessons learned from SharePoint. There is also a pretty seamless upgrade path. Power Pivot models can currently be promoted to SSAS, and the expectation is for the same to be possible with Power BI models (all the same backing database technology).
I hate to be so vague, but there's a lot to both products. I'd be happy to dive deeper into some specific cases if you've got questions.
Power BI as a standalone product is pretty brutally limited in terms of data volume (250MB compressed data model is the max that can be hosted in the cloud service), and is missing the extensibility and flexibility that comes from a tool like SSRS.
As a self contained product, Tableau will likely hold the lead for some time. As a platform, I think Microsoft is beyond Tableau - they cover far more of the BI spectrum (and well, especially with SSAS) than Tableau seems to ever intend to.
It all boils down to this, in my opinion: PowerBI is for dashboards and reporting mostly. Tableau can do dashboards, but it can also do "analysis". Uncovering insights in the data that go way beyond simple cross-filtering.
In my mind, they are different tools for different purposes.
Just because you can, doesn't mean everyone will rush to do it. This is a self-service platform. Putting the data in the hands of decision makers. There is hierarchy in how people use such software:
The user: He uses only the apparent features in the GUI. Like the grid and aggregation formulas in Excel. He learns how to use the software from other people showing him how to do stuff.
The Power User: He has deeper needs, but can only be bothered to use features 1 or 2 levels deep in the GUI. Like pivot tables, vlookups and index/match, logical operator formulas in Excel. He learns how to use the software from tutorials.
The Advanced User: He has a task and does not mind getting his hands dirty in order to fix it. Uses DAX and Cube formulas. Perhaps even Macros. He learns by googling his problem and reading documentation.
The Developer: Solves the problems at the programmatic level.
Tableau occupies a very specific spot. It is brilliant for the User who only consumes dashboards via clicking on them. No explanation needed and it is super polished. It is also powerful enough for the Advanced User who can perform relatively sophisticated analyses from the interface. Generally speaking, it is not a good fit for the Power User who doesn't have the need to justify using it. It is also not a terribly good choice for the developer because it is too restrictive and the programmatic features are not well thought out.