The Witcher 3: Wild Hunt has a ridiculous amount of content. It has hundreds of side quests, a huge main storyline and a giant open world. This will never leave you bored, even after completing the main quests.
The combat is ultimately very good. Generally, there are two different approaches to the whole thing. You can either run in swords swinging and slashing everything in your path, or you can use a more sensible approach by crafting potions, blade oils, learning your enemies weaknesses followed by lots of dodging, rolling and parrying. The latter of the two is normally the better approach. There are also signs, which are basically just magic spells. There are five different signs which are all upgradable: Igni, for burning your enemies, Aard, which is a simple knockback, but can be used for destruction, Axii, which stuns your opponent, Yrden to make a ring which - once your opponent is inside - slows them and makes them more susceptible. Finally, there is Quen, which creates a shield around the player, and is probably the most useful of the lot. There are some flaws to the combat though, such as the repetitive nature of it and the fact that in some areas it just feels a little unrefined. For example, you may be trying to attack one enemy, but accidentally swipe at another. This can become quite a nuisance, but is easily manageable.