Welcome, I've spent many a night hiking in the National Parks (US). I understand and appreciate the permit system, though somewhat frustrating in a last minute escape to mountains if your particular objective is subject to out of stock permits! You will love the 'freedom' that you will find here. Please do venture away from well trodden tracks and paths ( google earth will help you decide what is and what isn't well trodden), assuming your nav skills support featureless navigation. There is a real joy of navigating across pathless terrain, something I rarely had the chance to do in the US Parks since off track ventures can be really hard going (Deserts excepted!)
One aspect that I did appreciate in the system in high altitude fragile habitats was the 'pack everything , yes everything!' out mandate , with the odd ranger around checking that indeed your are carrying 'everything' out.

Andy

The only thing you’ll need to watch in your planning is that some of the Cairngorm passes (lairig Ghru - definitely and Lairaig an loigh - depending on how you access it) may need a FWA and all the summits will. The routes round the edge of the plateau are brilliant and well trodden by Challengers and don’t need FWAs.

Thanks all for your kind and very informative responses! And yes, I'll be sure to include any FWAs where necessary!