Come across a new word without immediately needing to translate it into Russian, no matter how long or counter productive the effort; pronounce the word opposite; stay on one topic for more than five minutes at a stretch; do an activity without lapsing into Russian; use articles correctly - or prepositions; find vocabulary in a text and work with a partner at the same time; remember the difference between choice and variant, economic and economical ("Buying supermarkets' own brands is economical," I explained to an adult business class. "Yes and very bad for your health," one replied.); turn their mobile phones off; hear the word test without grinding their teeth and running for the door.
UPDATE: A reply from a mate in the Czech Republic. Things his students cannot do:
2. Speak English.
3. Walk and chew gum at the same time.