John, I'm thinking you're drawing large generalizations from a small sampling.
First the following denominations are all kissing cousins with roots that go back to John Wesley:
Group 1 -- denominations that all are limbs branching off from the trunk of the tree of the Methodist Episcopal Church founded in 1785.
United Methodist, Free Methodist, African Methodist Episcopal, African Methodist Episcopal Zion, Christian Methodist Episcopal, Wesleyan, and a collection of independent Methodist churches.
Group 2 -- denominations that are grafts on other trees but that are all Wesleyan in their overall theology and often have similar ecclesiastical structures to the UMC as they are the product of people who left the Methodist Church to found new denominations without actually splitting off from the Methodists.
Church of God, Nazarene, Salvation Army
Group 3 -- denominations that may not show obvious Methodist connections, but nonetheless grew out of the movement which sprang from the Methodist Church such as camp meetings in the late 1800s.
Assembly of God, Pentecostals, Plymouth Brethern, FourSquare Gospel
Second, you just confessed that all the UMC you know are pretty conservative. Even in the south there are liberal churches are there not? Yet, in your experience those who attend UMC congregations are not among them. What does that tell you?
Sure there are liberal and conservative UMC members, pastors, congregations. Like with the country as a whole, you find more liberal examples in the northeast and northwest, and more conservative examples in the south. But you will find that pretty consistently, the UMC is at neither end of the spectrum relative to those around them. Having more UMC congregations than the number of Post Offices and McDonald restaurants combined, we're pretty much everywhere. Whereas other denominations are geographically defined, the UMC is not. Most people do consider the UMC mainline, because it goes back to the beginnings of this country and is so widespread. Until recently, the UMC was the biggest protestant denomination in the USA. Because of growth outside the US, it is once again the biggest protestant denomination in the world. We certainly were leaders in what was called the social gospel in the early 1900s. But I question whether that makes us liberal. Check out our actual beliefs, not what people feel toward us. Projections are just that, someone else's opinion gained apart from actual facts.
Third, rather than me tell you my opinion, why don't you read for yourself. Here are our doctrinal standards:
http://www.umc.org/site/apps/nlnet/content.aspx?c=lwL4KnN1LtH&b=5068507&ct=6466469¬oc=1