Female cats who are unneutered are more likely to get:
Pyometra - this is a womb infection that can be fatal, the cure is an emergency spay which is a lot riskier than a normal spay as the womb is full of pus. 2 forms, closed and open - cats have a better chance with open as the pus drains and is more noticeable to the owners. More common in dogs than cats - maybe cos it is easier for cats to sneak out and get pregnant
Mammary Cancer - 3rd most common form of cancer in cats, a very nasty cancer as it does spread quickly and although sometimes the tumours can be removed, it has a high incidence of recurrence. Most common age to get it is 10 and is believed to happen due to repeated unbred heats - would prob be more common if pyo didn't happen first. Can still happen in neutered cats if they have been neutered at a late age. Cats are 200 times less likely to get if spayed before their first season.
Ovarian Cancer - not heard of this a lot in cats, although I know a few people who have had older cats (found as older unspayed cats) spayed and the ovaries have been cystic so may have developed in time.
Uterine cancer - not heard of anyone with this, but it is one of the risks.
And I have read so many posts on cat forums of people with indoor only cats who's cat got out - my second foster was a 5 year old cat who had always been indoor only, got out once and got pregnant. The woman who had her could'nt cope with the kittens due to a relationship breakdown, so we got mum and kits, and mum went back after being spayed.
I am very pro spaying, as I took on a 14 year old rescue last year, and 6 weeks after getting her, found out that she had mammary cancer (that was a year ago today actually), and 7 weeks and 6 days after getting her, had to have her pts. I made a pledge when I found out that she had mammary cancer that I would do everything I could to promote spaying so that her death wouldn't be in vain. I wrote an article that was published in my local CP newsletter, and was posted on a few cat forums - will post it on here too.