Rear nearside wheel bearing rumbled badly. The replacement part was a ridiculous £80 because it's an ABS equipped model and apparently you have to buy the hub carrier too. And it was a nightmare to fit.
Wasted-spark ignition system failed. Turned out to be a common problem - a plastic clip that holds the 12V feed for the coil snapped, and the feed draped across a hot EGR pipe. Easy to fix, one fuse, some insulaton tape and a cable-tie.
Sticky idle speed control valve gives uneven idling in cold weather (but fine in the warm).
Rear drum brake cylinder started leaking. Easy job and very cheap to replace.
Blower fan switch works intermittently.
Water pump squeaks every rotation when the engine is hot. It has done since I bought the car, yet it doesn't leak a drop. I'm not too bothered as the water pump is on the ancilliary drive belt on the Mondy, not the cam belt, so if it siezes at least it won't break the valves.
Gears are very notchy, and it's difficult to get into 1st and 2nd when the engine is cold. In my experience of Fords this is due to the clutch release bearing sticking to the drive shaft and will ultimately lead to the whole clutch assembly needing replacement. I don't even want to think about how expensive that'll be.
Very rattly engine when cold. On the recommendation of several websites RE sticky values on MK1 Zetec engines, I started using thinner oil (15W5). This stopped the cold rattle.
Power steering has a little "notch" with no power assist when the wheel is centralised. This goes away after the car has run for a bit.
Uneven tire wear on rear. (Probably a result of the previous owner lowering the suspension).
The exhaust rusted through in two places, but was inexpensive to replace.
Having listed all the bad features of this car I do have to praise it's robust construction. It's big, heavy and solid, perfect for transporting the family. The Si also comes equipped with ABS and drivers airbag.
The power steering is rather too heavy and unresponsive for my liking, but the engine produces a reasonable level of grunt (136bhp) which is great for overtaking slow traffic uphill (or overtaking traffic anywhere, for that matter). But don't expect a great 0-60 time; at 1285kg it takes 9.5 seconds to get to 60mph, but the engine has enough power to give reasonable acceleration beyond 70mph if required.
The suspension is also very good on the Si with no body roll around corners and no skittering when bumps are hit while cornering at speed.
The fuel consumption is terrible when driven with a bit of gusto - I got a mere 24mpg. Motorway driving at a constant 70mph takes this up to about 38mpg in my experience.
The bucket seats are comfortable for long drives and the cabin is spacious. I'm quite tall and some cars I've driven leave little clearance between my head and the roof - the Mondeo has lots of room in this respect, and the seat height is electrically adjustable.