After various option permutations, I finally was able to inspect a text response with:

• Inspect URL content
• Monitor a change of the content
  I couldn't get it to work with any other combination except only and exactly those two.

I found the interface very confusing...
1. Checking "[URLTrigger] - Poll with a URL" w/o specifying anything else polls, but never finds any changes. Users should be notified that this is an invalid state, in the config, output, or both
2. [1] and checking "Inspect URL content" causes an error (see [4] below)
3. TXT/"Monitor the contents of a text response" seems especially misleading, since it requires a regex. Not knowing that (although I now see it in the help), I set TXT w/o a regex only to discover later that it was never finding any changes in the context. I think either:

• if no regex is supplied, assume all text (.*) - I like this one the best
• notify the user that TXT w/o a regex is an invalid config
• name it REGEX/"Something mentioning regex"

[4] Error output:
Error

Failed to parse form data. Please report this problem as a bug
JSON={"":"","builder":

{"command":"\"$JENKINS_HOME\"/build.sh","kind":"hudson.tasks.Shell","stapler-class":"hudson.tasks.Shell"}

,"core:apply":"","description":"","displayNameOrNull":"","logrotate":

{"logrotate_artifact_days":"","logrotate_artifact_nums":"","logrotate_days":"","logrotate_nums":"5"}

,"name":"Test Job","org-jenkinsci-plugins-urltrigger-URLTrigger":{"cronTabSpec":"* * * * *","urlElements":{"checkETag":false,"checkLastModificationDate":false,"inspectingContent":{},"password":"","proxyActivated":false,"url":"http://seandenigris.com/blog/?page_id=58","username":""}},"properties":{"hudson-model-ParametersDefinitionProperty":{},"stapler-class-bag":"true"},"scm":{"value":"1"}}

net.sf.json.JSONException: null object
at net.sf.json.JSONObject.verifyIsNull(JSONObject.java:2428)