The other day, while in a phone conversation, the old canard about open source being a "disruptive technology" came up. It's true, but I think it's one of those things (like "information wants to be free") that runs the risk of becoming a thought-cliché. You always want to talk about what's being disrupted, why, and to what end. For me, what's being disrupted most is complacency: the mentality that no one ever got fired for buying Vendor X.
Most of the time when people talk about open source as being disruptive, they mean it in a positive way. What's being disrupted is the old way of making software, where you build a black box, charge for it, and don't let anyone look inside. Open source moves the business model around a bit: Instead of charging for the software, you charge people for controlled access to expertise involving the software -- whether in the form of closed source add-ons or dual licensing, hosted installations, or 24/7 on-call tech support.
The bad kind of disruptive is -- well, the most obvious version is if you're someone whose proprietary work has suddenly been given short shrift thanks to the presence of open source alternatives. Bad for you, good for them. It's also an incentive to either make your product so good that people won't care it's proprietary (a tough mission) or change to a new business model. Why, for instance, pay for a proprietary database solution when there are many that probably do what you need without a price tag attached? Possibly because the ones that do come with a price tag often have something that simply isn't available anywhere else.
What's also being disrupted is software consumers' way of going about acquiring and using this stuff. They can pay for a proprietary solution, or pick up an open source one at no charge and run with it as is, or pay for a dual-licensed version and get expert support, etc. They have that many more choices, but more than that, they have the freedom to understand much more transparently what the consequences of each choice will be. It's not automatic, but it sure helps to have it.
To that end, the single biggest thing being disrupted in a good way, for me, is complacency about consequences. When you have that many more ways to choose (not just vendors to choose from), you have to think all the harder about the long-term effects of making those choices. Go proprietary and you'll have to think about the possibility of using a solution exclusive to one vendor. Go open source and you have to consider all the different ways those open source solutions can be consumed and implemented, from DIY to turnkey.
I don't see any of this as a bad thing, though. If you're forced to really think through your choices vis-à-vis software, that's that much more effort you have to put into doing everything well, isn't it?