Feedback, that sense of feeling the trail, the bike instantly responding to your every input.
Every half turn of the cranks propelling you forwards, the tyres telling you what they find through your hands and feet. The wheels turn and the tyres buzz, the frames silence only broken by the occasional chink of chain on stay.
I love hardtails, I have two of them, their characters may be totally different but the solid connection between rider, the trail and the sensation of riding that they provide are the same.
Convention says you should ride a hardtail first. Learn your bike handling skills before getting a full suss or risk having your lack of abilities hidden by a more capable bike. Whilst I agree with this to a point, riding a hardtail will teach you bike handling, and how to ride a hardtail whilst riding a full suss will teach you bike handling and how to ride with suspension.
Hardtails teach you to be smooth, to refine your line choice and control your speed. Riding smooth can be fast, but the smoothest line is not always the fastest line.
Full suss teaches you the fastest line, how to not brake and to trust the suspension. Trusting the bike to do its job is not always a short coming of skill. It takes confidence and bike handling to hit a section of roots and drops knowing the bike will work, and will work better if you don’t grab a fist full of brake.
Hardtails teach you about pedaling, climbing and body position, riding a full suss teaches you the same.
There is a place for both kinds of bikes, I am faster on certain types of trail on one and slower on others, it depends. Almost all my PB’s at trail centres are on hardtails and almost all my natural descents are on full sussers.
I wouldn’t pick one over the other but if I could only ride one bike for the rest of my life, it probably wouldn’t have a rear shock.