As always with HG Wells, what you admire first is the quality and prescience of his imagination. His vision of a fully urbanised, highly industrialised world, while in line with many late Victorian assumptions about the future, continues to impress; the specifics of his world – including aeroplanes and televisions – are perhaps more remarkable in their accuracy. Wells's literary foresight is equally to be commended, with The Sleeper Awakes providing a kind of generic bridge between the utopianism of William Morris and the dystopian satires of the more dissatisfied Huxley and Orwell. His world contains oppressed masses, totalitarian systems of control and plenty of misery: the key difference is that this misery doesn't lead to despair: Wells's masses are best characterised by their faith and hope.
This hope gives the latter half of the novel a feel that, in modern parlance, might be described as 'cinematic' or 'Hollywood'. The pace quickens as the world is redeemed; a slow, interesting exploration of Wells's vision is replaced with escalating action; the illusion is shattered. You can tell that Wells never finished the novel to his satisfaction: the narrative shift is unconvincing, and the second part of the book cluttered and marginally incoherent.
The narrative transformation also has ramifications for Wells's hero, whose initial confusion and wonder at the world in which he finds himself contrasts so effectively with the introductory chapters set in Edwardian England (in which the Sleeper falls asleep) and provides such an effective way for the reader to engage with the world. He becomes a messianic figure, and accepts this role swiftly and easily, despite failing to fully comprehend what it entails. Perhaps this is deliberate – a mark of the Sleeper's own faith and idealism – but it seems hurried and unsatisfactory nonetheless.
The Sleeper Awakes, then, finds Wells unable to fully realise his ideas across the length of a novel. He fails to add sufficient depth or breadth to his world. It needs time to breathe and grow which Wells denies it; the characters that inhabit it need space in which to seem like more than mere cogs in an allegorical clock. That said, I have no doubt that, had Wells been able to finish it, The Sleeper Awakes would not suffer from many off these problems, and the power of Wells's imagination would shine all the more brightly.