(My favourite Wyndham, for the record, is The Midwich Cuckoos. This is, I'm sure, because it felt closer to home than any of his other scenarios; it was very easy for 11-year-old me to equate the sleepy village of Midwich with the sleepy town of Wimborne Minster in which I was growing up. I also love The Chrysalids and Chocky.)
What are my findings? The story takes a long time to get going. There are mysterious events and spooky happenings, but I feel it takes too long for anything tangible to turn up. The language is pretty dry, and sometimes unnecessarily difficult; I had to ask Owen what 'morganatic' meant and I'd only heard of a 'clepsydra' because of klepsydra. Not that I mind extending my vocabulary, but neither word is at all relevant to the plot so I didn't feel suitably rewarded for finding out what they meant. There's also a fair amount of heavy-handed political satire as the West and the Soviet Union blame each other for everything.
These are the three factors I've pinpointed. All are present to a lesser degree in his other novels, though, and I'm still surprised that Kraken stuck in my craw. There are very few books I've started and never finished - and in a couple of bedtimes, at least this one can be crossed off the list.
As a footnote, my mysterious inability to finish the thing obviously continued to bother me throughout my teenage years. In my unfinished furry-comedy-scifi-children's-detective novel, Alcatraz Gatz - Space Detective (started when I was sixteen and lovingly migrated from Mac to Mac for a decade), there is an amphibian Chief of Police named Kraken Weekes. I was very pleased with this at the time; given that most of the other characters, planets and spacecraft took their names from Book II of the Aeneid, which I was studying for A level, it probably was the pick of the bunch.