Your first safari will almost never be quite what you picture. And the surprise usually isn't the wildlife. It's the pace.
The days start early, and they're long
You'll be up before six most mornings, because so are the animals. The light between six and nine is the best of the day for spotting and for photos, and by eleven most of what you came to see is asleep in the shade. Afternoons are for resting, and then you head out again around four.
People who haven't been told this arrive expecting a holiday and find a schedule. People who have been told plan a slower second half, or add Zanzibar, and enjoy the whole thing so much more.
You'll spend a lot of time in a vehicle
Tanzania is big. Between the northern circuit parks, you're looking at two to four hours of driving on a moving day, some of it on bumpy dirt roads. It's not wasted time, because you'll see plenty from the road, but it is time. A trip that squeezes five parks into six days is mostly a driving holiday. Fewer parks and more nights in each is almost always the better trip.
The guide matters more than the camp
If you remember one thing, make it this. Two vehicles at the same lion can have completely different mornings, depending on who's driving. A good guide reads tracks, knows which trees a leopard likes, listens to the radio and knows when to ignore it, and can tell you what an animal is about to do before it does it. Camps vary in comfort. Guides vary in whether you see anything at all.
Sightings aren't guaranteed
On a week-long northern circuit trip, most people see plenty of lion, elephant, buffalo, giraffe and zebra. Leopard and rhino are genuinely uncertain. A river crossing comes down to luck and patience, even in the right month and the right place. Anyone who guarantees you one is selling something.
So what actually goes wrong?
Hardly ever the animals. Usually it's one of these: not enough warm layers for a chilly dawn on the crater rim, a camera battery that died at four in the afternoon, or an itinerary with one park too many in it.