I thought the OP was a fairly open-and-shut case.
I expected ex-JWs who are now Christians of whatever denomination to offer some argument against, but I gotta say I'm a bit surprised from comments from posters I know are non-religious.
"Christians believe the trinity - JWs aren't Christians because they don't believe this"; "JWs aren't xtians because most don't take the bread and wine"; pointing out the fact that JWs themselves don't consider other denominations to be Christian; pointing out that JWs are big on following doctrine made by the GB - forget all this. It's mere detail.
Let's get a sense of perspective.
JWs believe the Bible to be the word of God, they believe Jesus died for their sins and they try to follow Christ within their world view. They obviously aren't mainstream but how can they not be a Christian group?