The glitches: http://img585.imageshack.us/img585/4344/7sk4.png
Without glitches: http://img43.imageshack.us/img43/1892/kue1.png

Here's the minimal code I could come up with, demonstrating the issue (comment/uncomment the "-fx-font-family: Roboto;" to make the issue appear/go away).

(It's written with the scalaFx dsl convenience, but this shouldn't matter here.)

{code}
package glitchtest2

import scala.collection.JavaConverters._
import scalafx.Includes._
import scalafx.application.JFXApp
import scalafx.scene.Scene
import scalafx.stage.Stage
import scalafx.scene.control.{Button, Label}
import scalafx.scene.layout.VBox
import scalafx.scene.text.Text

object GlitchTest2 extends JFXApp {

  stage = new JFXApp.PrimaryStage {
  
    width = 600
    height = 450
    
    scene = new Scene {
    
    stylesheets = Set("http://fonts.googleapis.com/css?family=Roboto")
    
      content = Set(new VBox {
          
          content = Set(
          
            new Button("Search") {
              style = "/*-fx-font-family: Roboto;*/ -fx-font-size: 11px;" // <= Uncomment to make the issue go away
            },
            
            new Text("Search") {
              style = "/*-fx-font-family: Roboto;*/ -fx-font-size: 11px;" // <= Uncomment to make the issue go away
            },
            
            new Label("Search") {
              style = "/*-fx-font-family: Roboto;*/ -fx-font-size: 11px;" // <= Uncomment to make the issue go away
            }
          )
        }
      )
    }
  }
}
{code}

https://docs.google.com/file/d/0B7lrDAvzeFVyZ09NeWdKMjh0aGM/edit?usp=sharing
=> The code archive containing this file + the gradle build dependencies (scala + scalafx) to launch it.

(Btw, is it on purpose that with modena.css, Text uses grayscale antialiasing while Label and Button use the colorscale (cleartype or smthg)?)

--
Edit: changing the system fonts (kde) doesn't seem to have an impact on the JavaFx default font used (so good chances are the bad anti-aliasing comes from this fallback/default javafx font).