I suspect someone started downloading the full blockchain off my full node.  Unfortunately, the effect was that my computer came to a screeching halt: moving the mouse and the pressing keys on my keyboard had absolutely no effect for several minutes.  When I regained use of my computer, I looked in the Task Manager and bitcoin-qt was using the most RAM, and a fair amount of processing power was being used as well.  I turned off bitcoin-qt and the problem went away; anecdotally, I'd say that's reason to believe that running the bitcoin-qt client was causing the problem.

So a little bit about my specs, both internet and computer: I'm using an internet connection of 25 Mbps down / 1.5 Mbps up - reasonable download speed (unfortunately I don't live in an area where gigabit internet is available), but not much on the upload front.  My computer has 5 GB of RAM, 250 GB of hard drive space, and a 3 Ghz processor speed.  The only part of my computer itself that I feel is underpowered is the RAM, I plan on eventually upgrading to 20 GB RAM, but haven't gotten around to it yet.

So here's my question: is there something about the way the bitcoin client is built that causes my computer to freeze up when people start to download the blockchain from me?  Or is this just a case of me not having enough upload bandwidth to handle someone downloading a full node from me?  I'd love to be able to support the network by running the bitcoin-qt client regularly, but this is starting to happen semi-regularly as bitcoin becomes more popular and I don't feel like my computer is THAT underpowered.