Tell me what I am doing wrong.

[gcs8]

This is what I have done and some diag output.

netsh interface teredo set state disabled
netsh interface ipv6 add v6v4tunnel interface=IP6Tunnel 23x.xxx.xxx.241 216.66.22.2
netsh interface ipv6 add address IP6Tunnel 2001:470:7:c71::2
netsh interface ipv6 add route ::/0 IP6Tunnel 2001:470:7:c71::1

Idx     Met         MTU          State                Name
---  ----------  ----------  ------------  ---------------------------
  1          50  4294967295  connected     Loopback Pseudo-Interface 1
12          50        1280  disconnected  isatap.{69841058-B8CF-47AF-BE73-820B1
659F51C}
21          10        1280  connected     IP6Tunnel
15          10        1500  connected     Local Area Connection 3

Pinging ipv6.l.google.com [2607:f8b0:4002:c06::67] with 32 bytes of data:
General failure.
General failure.
General failure.
General failure.

[broquea]

is 23x.xxx.xxx.xxx actually configured on your windows machine? If you read the NOTE under the config pastes, it explains what to do when the IPv4 endpoint isn't configured on the machine you are running the pastes on.

[gcs8]

Yes it is, it is one of the machines in a /28.

[cholzhauer]

Is your firewall passing protocol 41?

[broquea]

Yes it is, it is one of the machines in a /28.

If your windows machine is configured with one of the globally routed IPs in the /28 then either the Windows firewall is filtering Protocol 41, or your ISP is.

[snarked]

I try not to use Windows when possible.  When you did this:

netsh interface ipv6 add address IP6Tunnel 2001:470:7:C71::2

Did windows add a route for 2001:470:7:C71:://64?  If not, then the route for the next line has no route to the gateway.