Sitecore 9.1 is here, and folks will be installing it right away to play with new features such as Cortex, SXA 1.8, and JavaScript Services. Having installed Sitecore 9.1 recently, I've had a chance to go note the differences between the 9.0 and 9.1 installation process.
As always, start with the official Sitecore 9.1 Installation Guide on dev.sitecore.net. Installing 9.1 is mostly the same process as 9.0, but there are a few additional and updated prerequistes:
Sitecore Install Framework (SIF) 2.0
SIF has been updated to 2.0.0. Sitecore 9.1 cannot be installed with any version 1 SIF script. Sitecore also provides new install scripts for single-machine Experience Management (XM) and Experience Platform (XP) single-developer machines, which will simplify setting up development environments.
SQL Server 2016 SP2
A slight bump in database requirements (download SP2 here) - although compatibility with SQL Server 2017 (here) is even better.
Solr 7.2.1
Sitecore 9.0 specifically required Solr 6.6.2 to function out of the box, but 9.1 bumps that to 7.2.1, which is a major version bump for Solr.
.NET Core 2.1 Runtime
The new Sitecore Identity service - which provides native federated login capabilities and slightly changes how Sitecore login works - is built on .NET Core, and requires the latest runtime to be installed. Over the next few years, more and more pieces of Sitecore will be built on .NET Core, so this is a worthwhile prereq. Get it here.
SQL Server tools
There are a few new modules and tools that come from various sources. First, SQL Server PowerShell module (here) is a quick install. Second is the SQL Server ODBC driver (here). Finally, the SQL Server CLR types pack may already be installed alongside other prereqs from 9.0 installations, but if not, you can grab it from the Web Platform Installer.