I think it really depends what features you are talking about.
Feel of combat - you can quantify this by going back and looking at the old games and having a look at what types of feedback they gave, what the animations looked like, how fast it was, how long characters had to wait between actions, how long combats took in general.
I could go on and on, but I really think it's not that hard to see what people mean when describe something as feeling like the IE games. Josh Sawyer also had a powerpoint page on his GDC Presentation dedicated to the IE feels.