Be prepared for the hot and humid conditions. Nothing ever seems dry!! On the contrary, it does get pretty cold up on the mountain, so warm clothes are a must for the overnight stay at the mountain hostel and for the summit walk.
Some of the accommodation is very basic with communal (and often cold!) showers, although everywhere we stayed was clean. That said, some people were very unhappy with the state of the hostel (Gunting Lagadan) on Mount Kinabalu, although the one I stayed in (Pendant Hut) was fine.
It's an active holiday with lots of early starts and a lot of packing and unpacking, but if you plan carefully, this can be refined to an art form!! Don't take too much clothing - travelling light is the way forward. DEET is an absolute must everywhere (except on the mountain) - I used a ton of it and managed to avoid getting bitten. Watch out for leeches also - they don't care about DEET, but do respond well to being removed with a good flick!
Don't underestimate the Mount Kinabalu climb - it is tough, both physically and mentally! You do need a good level of fitness and a lot of stamina!! The final days on the beach are perfectly timed (and necessary) after the exertion!!
I really enjoyed this holiday and would highly recommend it. It fulfilled all expectations, although I would say I expected to see more wildlife in Mulu National Park. The second jungle stay in Kinabatangan more than made up for it though. We were also very lucky to see about 10 orangutans at the sanctuary in Sepilok. Due to the time of year we went (last week of October, first of November), the rainy season was beginning and we had rain every afternoon. It didn't stop us from doing anything though or spoil the enjoyment - even the rain was warm!! We were exceptionally lucky with the weather on the Kinabalu climb though, with perfect conditions on the summit climb.
Go to Borneo, enjoy it, have fun, but be aware of the ecological issues and problems arising from deforestation and do your bit to help! I adopted orangutans at Sepilok and became a Friend of the Rainforest :-)