1. I wouldn't need to hire an extra IT person. I already have very capable Linux admins.
2. Bad Windows sysadmins are less expensive than good Linux sysadmins. I agree with you there. Good Windows admins, however, are just as expensive. Trust me...I've hired quite a few.
3. Again, I have well-trained Linux guys on staff.
4. This is a very valid possibility we'll need to consider.
5. Not worried about that at all. We use a lot of open source without commercial support options, and our experience with Microsoft support is four hours of scripted troubleshooting on average, with about a 60% success rate of resolution.
6. Not concerned with this at all. Samba has been here for a long while and I don't think it's going anywhere.
But IT departments are expensive, why can't we just hire kids without college degrees to work minimum wage and make millions of dollars of hardware sing in harmonious chorus? The kids are good with the computers right?
1) It's probably not enough money to hire even one extra IT person to deal with new issues that will arise with Samba 4
2) Linux sysadmins are more expensive than Windows sysadmins.
3) Cost of retraining existing IT staff.
4) Potential loss of productivity of your 2500 users
5) Lack of commercial support options.
6) Uncertainty about the future of Samba in general (release time tables, feature support, etc)