I'll bite. I define it as the ability to be expanded beyond what Apple ships.
In my case, I started with a 2009 Mac Pro with 3.33GHz quad CPU, 3GB RAM, 4870 GPU, Apple RAID card, and four HDDs consisting of a 640GB and three 1TB disks.
Before booting for the first time even, opened it up and expanded the RAM from 3GB to 16GB. I ran it this way to edit a feature-length movie shot on P2 cards, which it handled easily.
Then I started getting more work with different types of footage, to the point that I needed to expand and upgrade further. I changed out the 16GB RAM to 32GB, changed the GPU to a GTX 285 and then a 5870, and changed three of the HDDs to 2TB for a 6TB RAID 0 for a short time.
Since that wasn't solving all my problems, I sold off the Apple RAID card and expanded my RAID with an Areca RAID card and external 8-bay box, filling it with eight 2TB WD RE-4 disks, and setting that up as a RAID 6. Then I swapped out my quad core CPU for a 6-core CPU after running the firmware update tool. I added CalDigit's eSATA/USB 3.0 card for connecting all my client's eSATA and USB3 drives. I added an LG Blu-Ray burner, and replaced the original 640GB HDD with an SSD. I added a Voyager Q dock for the ability to drop any HDD or SSD into it with ease.
I went from geekbench scores of ~10,000 to ~16,000. I went from disk throughput speeds of ~330MB/second to ~800MB/second. I went from 3.64TB to almost 40TB of connected disk space. I'm only using 8 of the 16 ports on my RAID card, which means I can expand to another RAID box of 8 disks, should the need arise.
To me, that is upgrading and expanding, and a good example of proper expandability. As far as problems... I did some research, asked some questions from awesome people here, and avoided problems. I don't fear closed systems such as the iMac... I just avoid them.
My Mom loves iMacs, but hates that she has to buy a new one every three years. The first two she had died almost exactly three years from purchase. She still has the second one with a fried logic card, hoping I'll fix it one day. I told her the same thing Apple told her... that it's a worthless P.O.S. which is better replaced than repaired. It just drives her crazy that she has a whole iMac that probably would work fine if repaired. Eventually, she'll tire of seeing it sit uselessly on her desk beside her 2011 iMac, collecting dust.
If one wants to expand or upgrade an iMac, one must accept the limits and pain-in-the-anus factor involved therein, as we all know. Some might think I have fear of closed systems, but I consider my attitude to be wise enough to identify iMacs as disposable systems for those that fear expandability.
The fear is elsewhere, not here.
