So, a mystery story that is all clues and no description would be like a laundry list: Discovered, (1) one beryl coronet missing (3) three beryls, in the (2) two hands of (1) one Arthur Holder. No one would read such a story. So one way that Conan Doyle slows down the reader to make him or her pay attention to what's happening is to throw in passages of dense imagery. Consider the atmosphere wrapped up in the following passage:
The equinoctial gales had set in with exceptional violence. All day the wind had screamed and the rain had beaten against the windows, so that even here in the heart of great, hand-made London we were forced to raise our minds for the instant from the routine of life and to recognize the presence of those great elemental forces which shriek at mankind through the bars of civilization. (Orange.3)
As the beginning of an unusually grim murder mystery for a Holmes story, this use of personal and introspective observation is particularly effective. Watson's narration doesn't wander off into these passages of thick description very often, which makes their impact all the more powerful when he does.
The tone of Watson's narration is also suspenseful. Suspense grows out of the awareness that we, the readers, know less about something than certain characters in a story do. And we're always going to know less than the great Sherlock Holmes. As a first person narrator, Watson helpfully draws attention to what we don't know by regularly pointing out what he doesn't know: "It was obvious to me that my companion's mind was now made up about the case, although what his conclusions were was more than I could even dimly imagine" (Coronet.168). Or "What a tissue of mysteries and improbabilities the whole thing was!" (Valley.104). Every time Watson exclaims over how little he understands, he emphasizes the huge gap between our knowledge and Holmes's – something that only increases our need to know how it's all going to come together in the end.