Generally: the industrious, innovative, charming, friendly people; the fresh and delicious food; the thousands of motor bikes / scooters (and associated polution!) often used to transport anything from families to crates of fruit, veg, livestock (pigs, hens), etc; the culture of sitting on tiny plastic stools or squatting flat-footed on the pavements to sell, cook, eat, socialise, etc outside their houses; the longlasting effects on the countryside and people from the Vietnam/ American war.
Specifically: Halong Bay was spectacular (although the weather was grey, cold and damp!); Hoi An was charming and we had a lovely bike ride along the river, around the town and to the beach (too cold to sit on the beach though!); the peacefulness of our stay in a longhouse at Lake Lak; the vastness of the Mekong Delta and the way of life.
The weather was (unusually) cold in the north until we'd crossed the mountain pass after Danang when the air started to feel warmer. Therefore check the weather on-line before departure as most of us had not packed enough warm clothing. I ended up with a heavy cold / congestion due in part to pollution in the cities and the piercing cold wind.
Sheet sleeping bags are widely available in Hanoi for about $5 (pure silk costs a bit more) also North Face rucksacs, travel bags, etc and Kipling bags are very reasonable.
It really helped to have a bit of knowledge about Vietnam and the areas visited before the trip.