Since I haven't yet made the leap to residential VoIP either, here are mine:
1. VoIP services don't support my home alarm system or my TiVO, so I would need to maintain a POTS line.
2. I have Comcast as my broadband provider, while reliability has been good for the last year, it's been downright awful for the last two weeks. Meanwhile, the last time I lost POTS service was during hurricane Gloria in Long Island in 1985.
3. Those I know who have tried VoIP, generally using Vonage, have complained of spotty reliability and poor customer support.
4. I have this nagging fear of what happens to my home phone number if my VoIP provider goes bankrupt. What happens if it gets lost in cyberspace and can't be transfered (we need a system like Network Solutions/Verisign offers to allow web management of URLs - I should be able to manage my phone number through a web site).
5. Given #1 above, and the fact I make little to no long distance calls from home, the cost benefit just isn't there. Since I still need a POTS line, I'm looking at about $20 a month for a local line, plus $25 per month for a VoIP line, meaning that my current phone bill, which is currently $49 including unlimited local and long distance, would be reduced by about $4.00 (I'm not including taxes & other fees in this equation). Saving $4.00 to me doesn't offset the negatives I noted in the other points above. I should add that I get $5.00 off my wife's Verizon cell plan for bundling my home phone service into the same bill, so the cost savings equation goes away.
6. (Bonus reason) I just don't have a need for anything more than caller ID and call waiting for my home service. I'm not going to set up any custom features via a web site for my home phone.
7. (Bonus reason #2) I'm not willing to trust the safety of my family to VoIP. I'm extremely concerned about the ability for residential services to provide the same kind of 911 support currently provided by POTS. I want to be guaranteed that calls are handled the same way, and aren't sent to different call centers as has been discussed with the Vonage/911 lawsuit in Texas.
8. (Bonus reason #3) I might be wrong about this, but I don't think any of the VoIP services support faxing.
A couple of disclaimers. I've used a public VoIP service in my office for over 3 years now, it works great and I have no complaints, but they deliver it over their own T1, any software update requires us to walk around the office and reboot each phone, and the original service provider has gone bankrupt. We also can't support fax over our VoIP service, we needed a POTS line for that.
I'd also note that I use Skype a lot, I like it but the call quality is nowhere near as good as my home phone or my office VoIP line and the client seems to be somewhat unreliable. We tried a three-way conference call yesterday with two PC users and me being the lone mac user. My client crashed on startup, and one of the other user's clients crashed several minutes into the call.
Now having said all that, if I needed a second home phone line for work, I would select a VoIP provider in a heartbeat. I might also look at a VoIP service if I made a lot of long distance calls. I'm just not convinced that replacing a working, cheap POTS line with a more unreliable VoIP line makes any sense right now.