Not much.
I've had a bit of trouble with the airbag symbol flashing on the dashboard, so I took it to the dealer where I got it and he said it was because all the wiring for the airbag was under the front seats and when they are moved around a lot, it sometimes dislodges them.
The symbol is annoying, but seems to disappear after a while. Strange.
My Westcoast had a stainless steel sports exhaust and K+N filter to aid engine performance, but sometimes it feels a little sluggish. I don't know if this is the same in all Westcoasts or just mine?
All in all though, haven't had many problems.
When I thrash my car the engine opens out and the ride gets smoother and the car gets quicker. It does surprise me sometimes. And it's a very good surprise!
My Westcoast has VTR seats in it and they are very comfy, so I couldn't really comment on standard Westcoasts seats, but the interior is fair.
I think improvements to the car could be a drinks holder that actually holds the drinks (there are two cup grooves on the lid of the glovebox, but try turning a corner or accelerating too quickly and you're in trouble!)
Back end speakers would be good as standard.
West coast seats seem to let you slide across the other side of the car while going round a bend. I would recommend upgrading the seats to VTR or VTS.
Drive-shaft is seriously in need of repair, started to knock at 48,000miles gradually getting worse.
Started spitting out oil for fun around 56,000miles.
Rear body panels come loose, a problem I've spotted on a few saxo's recently.
Front tires don't last long due to the car loving to wheel spin at any time especially leaving junctions.
Great performance for a little car though!