I think it's the dressing.  The closest I have found to the dressing you get in restaurants is balsamic and olive oil dressing.  You can buy it in supermarkets but, here is a recipe I am going to try.  I have made something similar before, without the mustard and shallot and it was pretty good, especially with some ground up garlic added.
Ingredients
Method
- Pour all of the ingredients into a jam jar for which you have a lid and screw the lid on tightly.
 
- Shake the jar vigorously for 30 seconds, then taste and add a drizzle of honey or agave if you prefer. Store covered in the fridge for up to three days before using.
 
- Shake the contents of the jar again just before pouring over your salad.