The shared differentiator was industrial design. Sinclair spent a small fortune on making products look distinctive. The most successful designs (ZX81, Spectrum, QL) were done by Rick Dickinson, but you can already see it in the sleek black plastic Sinclair Executive and the flamboyantly gold plated Sovereign from the 70s.
All of these products were designed to look exciting and futuristic.
The internals were penny-pinched to a ridiculous extent, and were usually flawed and often not all that functional.
But the externals won design awards and caught the imagination of ordinary non-technical users, because they looked like nothing else on sale at the time.