I've not been able to figure out an alternative to cucumber that has the same flexibility and readability.

For example, I've not a clue yet on how to improve on this example:

  Feature: DuckDuckGo Search
    It should continue to kick ass
    And I should be able to search

    Scenario: Homepage performance
      When I go to http://www.duckduckgo.com
      Then I should see the page downloaded in less than 0.5 seconds
      And I should see the page assets downloaded in less than 2 seconds

    Scenario: Search over SSL
      Given I go to https://www.duckduckgo.com
      When I fill in "q" with "site:news.ycombinator.com"
      And I press submit
      Then I should be on https://duckduckgo.com/?q=site%3Anews.ycombinator.com
      And I should see "Hacker News" within "h2"

The problem is that this looks tempting, but what if I change:

    Then I should see the page downloaded in less than 0.5 seconds

to

    Then I want the page to download in under a second.

The natural English looks very tempting, but the actual terms you can use are very restrictive.

Thinking monitors.txt would need a validator, plus the monitoring providers can alert customers when they've entered something invalid.