NOTE: I am not a Doctor so please excuse the spelling
1. High blood pressure
No problem as long as you are controlling it with a daily tablet or diet or exercise
2. Any heart related issues including requiring a stent being inserted. Grounded for 12 months, then a report from your Cardio Surgeon/Specialist is required annually to be submitted to the CAA medical board for revue before they will give you a medical. But there are many people who have had heart attacks and gone back to flying.
3. Stress ECG, not required for Class 4 medical but highly recommended as it is a quick and easy way of early detection of heart disease. He told of a case of a 40year old who 2 years ago was clear and then this year the ECG picked up a slight variation. Dr Stoner immediately sent him to a specialist and they inserted a stent. He says 100% this guy would have had a massive heart attack if they did not pick it up.
4. Diabetes. If you are injecting insulin daily then absolutely no flying ever. The other type i.e. taking a daily tablet or diet/exercise to control it you can fly. He says diabetes has become a very common disease.
5. Mental Illness. Obviously the serious illnesses like Schizophrenia, Bipolar disorder etc would exclude you, but if you are on any form of Anti Depressant then you must be grounded. Most of these courses last about 6 months and then you must be weaned off them. It's not when you are on them that's the problem but the sudden unpredictable personality shift when you miss a tablet or stop taking them that is the big worry. So you will typically be grounded for a year. He talks about a Japan Airlines jumbo crash where the Pilot was suffering from depression and committed suicide, taking 380 people with him.
6. Head trauma. If you have had any accident where you lost consciousness, suffered concussion etc you must tell your Dr immediately. They will then have to do a brain scan to be satisfied that all is well. Any injury resulting in skull fracture, or bleeding on the brain etc will ground you for life. This is because you now stand a very very high risk of developing Epilepsy at any time for the rest of your life.
7. Asthma. If you have to use an inhaler like Ventolin you will not get a medical. Asthma is also exacerbated by stress and we all know sometimes the cockpit is a stressful place to be. Additionally Asthma sufferers have a lower oxygen level in their blood and will suffer from Hypoxia much quicker at altitudes as low as 8000ft. However there are prophylactic preventatives such as Seravent that is taken once daily that fully controls asthma and you can get a flight medical if you are on this medication. You can see the difference on the lung capacity tests and if high enough you will be passed.
8. Eyes. No problem if you wear glasses. If you have particular bad eyes sight that leaves you nearly blind without your glasses you will be required to always carry a spare pair of specs with you. You can also wear contact lenses now without an issue.
9. Colour blindness. No problem but only VFR and no EFIS type systems in the plane.
10. Cholesterol. It won't ground you if higher than 5 but for your own health it is better to control it. Drugs like Lipitor are widely used with almost no side effects.
11. Cancers
11a. Prostrate - will not ground you because it is not a particularly invasive type of cancer. He mentioned a PSA test(

11b. Melanoma. This will ground you for life. He says anyone suffering from this form of cancer stands a very high chance of brain cancer, tumours etc.
11c. Brain tumour. If operated on and successfully removed will ground you for 5 years. After that a brain scan, tests, report from specialist etc will have to be sent to the medical revue board. They might pass you but you will have to submit these test yearly or even more regularly
12. Donating Blood. If you donate blood you need to be grounded for 24 hours to allow your body to replenish itself
13. Anaemia, grounded.
14. If you have bleeding piles/hemorrhoids etc you will be grounded until treated. Apparently this is one of the fastest ways for your body to lose blood and not be able to replenish it quick enough.
15. Ears. Bad hearing will not ground you, you can wear hearing aids or crank your volume on the intercom.