Generally small scratches etc are not charged for. When you take a lease, before you confirm, you get the T&C's which show you whats allowed and whats not. Small scratches etc are ok, anything that you would normally repair anyway should be fixed before sending back
Generally tyres need to be legal before sending back, but hey, they should be anyway.
Mileage rates, check the extra charges for going over - some state Xp/mile but after a set amount the pence per mile goes up higher, just be aware.
It may be worth getting GAP insurance to cover the full value of the car should anything happen. Its not a lot, usually under £100 for 2 years
For normal insurance purposes the rules vary but most leasing companies will be the registered owner. Most of the big insurers are used to this and it wont matter, but worth remembering
Generally the process is very simple and the car will be delivered to you and picked up when the lease is done. Many will offer the chance for you to buy or extend the lease at the end.
Servicing as above, will be done to you unless you get maintenance but most new cars will only need 1 or 2 services depending on the term taken.
Don't pick a car you want, be totally open to something that meets the needs. Look for a good deal and not a specific model and you will save much more money. Be patient too as deals change every month or so. Many of the better deals are on "stock" cars.
That pistonheads thread is worth a trawl through to give decent info on deals available. Some deals don't exist, companies can be a bit ropey to draw in the customers.
A lease isn't for everyone. I like them, but only if its an amazing deal. Many people don't like them as you never "own" the car, but for me its all down to the total costs over the term for a car that meets the requirements. Run the calcs on a spreadsheet and total it all up.
Note that deposits are where most of the deals are made to look "cheap". Most are 3, 6 or 9 months up front but some resort to a larger number for the deposit to make the monthly charge look lower. Always just total and then divide it across the term to get the real monthly cost of course. For me a 6+23 suits me best.
edit: added more info