I do lots of experiments in Sinatra and some of those use Sequel (when I need a database). I think Sinatra has some rough edges, and it surprised me how I needed to initialise a Sinatra app:

    module MyApp
      class Web < Sinatra::Base
        def initialize(app = nil, params = {})
          super(app)
          # setup stuff
        end
      end
    end

Also, I also find it sad that people never seem to know that they can use mount command in Rails where you can mount random rack-apps in your routes, even though most people probably do use it with some of their gems, here it is with Sidekiq:

    mount Sidekiq::Web => '/sidekiq'