The BDS course itself- a very long, rambling, highly academic education (not really training) that is almost like going back to school again. You'll likely make great mates as I did, but most of the courses across the UK have nowhere near the level of clinical contact required to turn you into a competent and confident clinician.
You will emerge from the 5 years unable to cope with anything except the simplest of clinical scenarios. Twenty years ago the course lasted 4 years and 1 term. There was no VT year. You could start normal general practice almost immediately. Today it is 5 full academic years and they are talking about making VT 2 years as they feel students still do not know enough to practice independently. I think you get the message.
Clinical sessions are like gold dust and some students at a certain London dental school are in uni TWO and A HALF days per week in their final year. You can leave uni with only having taken a few teeth out and done a few fillings. You will however, leave knowing absoloutely tons of theory about things that your professors are interested in (e.g. tooth wear) which no one in the real NHS world has the time/money/expertise to do anything about. The other thing to remember is that there are obviously loads of exams. How many depends on your individual school. But, the one I went to had them all in 1 week in November, so really you didn't need to open a text book for almost the entire year lol.
The relationships with tutors are challenging. If someone takes a personal dislike to you, they can fail you at their discretion which makes life horrible while you wait to find out if you'll be allowed to proceed to the next stage. As a result, there's loads of backstabbing, sucking up to people you dislike and near constant stress.
Of course, learning all this new stuff (and the practical work) is a novelty and can be interesting and fun. But training to be a dentist is like climbing Mount Everest and the feeling you get when you open the envelope saying you passed your final exam is like no other.