It wouldn't be surprising if over 99% of visitors go to Leighton Moss for birds, with the remainder going for dragonflies, but I'm afraid I tend to find this sort of birding boring. On the whole, I'd much rather have the birds up close and personal, even if it is just European starlings on a bird feeder. The one exception to the general reserve experience was when I visited Inner Marsh Farm for work on a day when it was closed to the public. On this day I ate my lunch in the hide in the absence of birders (who are a funny bunch) and had water rail and other waders literally metres away.
What excited me about Leighton Moss was the flora and the invertebrates.
One of the things that I've known about for a long time, but saw for the first time, was the alder moth caterpillar Acronicta alni (though I will admit that it was my brother that remembered the name). The alder moth is a widespread species, but the caterpillars are rarely seen, as they apparently spend much of their time in the canopy of various tree species, on which they feed. However, the final instars of the caterpillar are fantastic, looking like something more suited to the tropics than wet-and-dreary UK.
The other rather special observation was the bug Pictomerus bidens feeding on a caterpillar. I had seen this species before in Cornwall, but had never found it feeding. It actually came as quite a surprise, as I had not realised that it was carnivorous!