Waterpump (ideally needs a new one with metal impellor). Fails typically around 30k. Common VAG problem, not restricted to Seats.
Bosch DV is crap generally, Forge do the 007p to replace it, it will last longer. I still have the Bosch, seems ok, but then mines not chipped yet. Bout £25 for a Bosch or about £90 for a Forge. Common VAG problem, not restricted to Seats.
Door seals, not a common problem but do check the carpet for dampness. This issue may have been fixed before the R came out.
Coil packs, these have been revised and have a 5 year warranty from when the car was new. Depending on the age, it may have the new ones already. Bout £25 each. Keep one in the car, it's easy to change and the AA etc do not carry them as spares. This is a common VAg fault, not restricted to Seats.
MAF, these have a fairly high failure rate. They usually last a long time until you fit the wrong type of induction/air filter. You MUST have a dry filter system (sponge) because oils and other contaminents will kill the MAF. Again, common VAG problem.
Paint on Brembo brakes can fade to pink if acidic cleaners are used. Also if brake fluid is spilled on them (as often happens during services). The paint is not covered under warranty.
Stonechips, as with most modern car is a problem. Most cars have water based paint and they can chip easily. Worst colours are black and extreme blue. Chips in black can be seen easily, Blue chips the easiest of all colours.
Recommended tyres are the Eagle F1 GSD1's. Expensive and don't last all that long. Worth it though.
Brakes and pads are expensive! Keep that in mind.
Service costs aren't too bad, the 40k one is one of the more expensive (about £300 i think).
Should average about 32 to the gallon, as low as 15 witha good thrashing. Have seen 40 to the gallon on the M/way (with aircon on).
If you get one, get VAG Com. You can enable a few features such as Auto Door Locking.
Cruise control can be added if you find a decent dealer. Costs about £300. You can do it yourself, you need a stalk with the controls, the loom and Vag Com.
S3 strut brace can be fitted, if you remove the engine cover.
Revo or Jabba offer the best chipping/tuning.
Blueflame or Miltek are recommended for exhausts.
Rear suspension top mounts can rattle. If you give them a blast from under the rear arch with a jetwash, it may stop for a while. This can be done under warranty.
There are some common squeaks and rattles. They all usually have easy fixes.
You can get rear spacers to improve the look of the back wheels. Makes a cosmetic difference only, but does help the looks.
Will keep adding as I remember.