I'm both a serial renter and a landlord.
I can see both sides (and my kids have only ever been able to decorate a room once. To the extent that these days, their rooms basically resemble cells because they don't see the point of choosing things for a room that they won't have for very long). The trouble is, that as a landlord, I've had tenants do some monumentally stupid (and in some cases downright life-threatening) 'improvements' to the house I rent out - so now I'm careful what I approve. As a rule, I'm actually OK with some redecoration, as long as it's either put back, or not completely hideous. I do find people banging holes in the walls to hang pictures a bit frustrating though, because I've seen people make a terrible job of it. As a landlord, I fix things promptly, I maintain what needs to be maintained, I understand that accidents happen and I replace with good quality rather than B&Q's cheapest etc. I make sure that the heating system is efficient and modern etc.
As a tenant, I expect to not have to muck about fixing plumbing, or have tatty decoration - and as a rule that's been the case (although not where I am now). I've rented in a lot of countries, and I'd say the UK is towards the top in many ways (if you have a decent agent/landlord) - other places I've basically been expected to treat it as my home - do all the maintenance and repairs. Or I have ancient hot water/heating systems (like now) which cost a fortune to run.
I would like to see some kind of standards for rental properties - much like there are for housing association properties - landlords should be required to upgrade houses to be energy efficient etc (simple stuff, like thermostatic radiator valves make a huge difference). I do think that longer term leases should be more widespread (I actually think the agents are part of the problem here - persuading mine to let my tenants have a 5 year lease was a bit of a battle), and I think that if you're on a long term lease, it should be more the norm to be allowed more freedom in the house regarding superficial things like painting the walls - after all, if the expected life-time hall re-painting is 5 years, and your tenant has a 5 year lease, then you've lost nothing as a landlord by letting the tenant repaint how they like (oh, and assuming they use the right paint.. looking at a tenant who used wall paint over an entire side of a room - radiator, skirting boards, and all..)