Cutting my teeth on Exchange 2013 means I had it easy compared to the days of old. As a project I had to go back and stand up a brand new Exchange 2003 environment in my lab.
To be fair this was not as painful as I had imagined.
There were some leftover ideas from days gone by that immediately stood out to me such as having to change from disc 1 to disc 2 during the installation and schema update, the overall design of the interface, the lack of CLI tools, and good old /exchange.
My first true test, which I failed, came in the form of SSL and not knowing the way 2003 works. 2007 and up comes with a self signed certificate when using https, 2003 does not. I spent 2 days trying to figure out why https would not work on my /exchange directory until I realized that I either needed to stand up an internal CA or get a trusted 3rd party cert. Here is a link that shows my pleas to reddit for help (spoiler: not the first or last time this will happen)
I ended up going with a 3rd party SAN cert with the names domain.com, mail.domain.com, autodiscover.domain.com, and legacy.domain.com in preparation for what is to come.