Oh gosh, don't even start us on the perfect pushchair! I think you could write many essays on each part of the pushchair let alone a whole one!
But let's try! If I tell you about problems I would be here all day so here's what I would like with ome reasons.
Seating - has to be comfy - well padded, the child shouldn't be able to feel any of the chassis under them. Has to have the right depth / knee / width/ height / leg proportions with a good foot rest. There are a lot of good pushchairs which just have something wrong with the proportions (I can give you examples if you wish). Rotating footrests allow for baby legs to stay level and for room for toddler legs to grow. The child should not be able to drag their feet on the floor, nor have it's knees in it's face!
Should have a choice of outward or parent facing with a good easy to adjust recline in both directions. The seat fabric should come together nicely as the seat lifts up if it's a toggle recline (rather than a rotating recline).
Large hood - fitted well ie no gaps at the back or sides. We live in the UK so winter warmth IMHO is better than sun protection / ventilation but visors are nice (I like the Easywalker Qtro visor that has toggles to hold it out of the way).
Harness - should fit a newborn and 3/4 year old with really easy adjust so that you can swap your children around easily. The buckle should be hard for a child to undo. Ideally putting one side in and then the other, not both sides together.
Adjustable handlebar - is really essential IMHO. Soft to the feel, the same diameter as the average hand, but must not easily spilt or damage.
Large basket - mums like to / need to shop. The more storage, storage pockets, bottle holders the better including space to put or hang toys (pockets or toy loops in the hood maybe?) In built hooks for keys, bags etc.
Bumper bar - swing away and easy to remove without it falling off with a protective cover on - or even a toy / drink pouch a bit like the Baby Jogger Console but better!
Wheels - forever air tyres - air tyres but without punctures.
Good brake - handbrake like the Baby Joggers
Suspension - good suspension but not too bouncy - I like to lean slightly on a handle and for it not bounce away! The front wheels shouldn't crumple as you go off a reasonable sized step.
Fold - as small as you can get it in one piece. Bulky is not good. The fold and unfold should be easy. Not lots of steps and you shouldn't need an instruction manual to do it.
Be wipeable or easy to clean.
Peekaboo hood windows with magnets.
Be suitable from birth and as comfy for a newborn as it is a 3 year old....and preferably fit be tested for a four year old.
Should be simple to manoeuvre one handed even with a 15kg child in it. It should not veer to one side when pushing. Weight balance is important.
I think most pushchairs have niggles but there's one called the Baby Jogger Versa which has just been shown as a preview which is getting near perfect if you want to look it up. We will have to see how perfect it is when it is launched.
Good luck.