Everyone's different OP. Our DS (first child) is 14 weeks and our best things have been:
Perfect Prep (DS was FF from birth)
Mam bottles and dummies
Muslins
Chicco Next 2 Me crib.
LEKA baby gym - £20 from IKEA. We have it over the changing mat and DS adores it. Makes changing very easy.
A lie flat car seat (we have the Joie one). Expensive, but means we don't have to worry about the length of car journeys.
Things we haven't got on with:
Bouncer (the same Joie one that a PP linked to up thread!) DS screams every time he's put in it.
Slings of all types - we always thought I'd carry our baby in a sling a lot but he's huge and I'm small, and we both hate getting hot. Slings just don't work for us and DS loves riding in the pram, so we do that.
I also bought too many clothes. I didn't realise that all our gifts would be outfits (no one bought toys, books, or blankets for DS when he was born).
As you see, my two 'don't bother' items are several people's essentials! I think some 'mistakes' are inevitable, because some of it depends on the baby. Most babies love bouncy chairs - DS is a bit weird in that respect. And it's possible that if we have another DC we would have a little dainty one who hated the pushchair and only wanted to be carried...
We haven't needed a monitor yet - might buy one when DS goes into his own room at 6 months, but for now he's never on his own so not necessary.