Like Ko said, as technology gets more and more advanced, the reliability, consistancy, and durability increases. Depending on the PSU that you bought in 2007 depends on how it stacks up with todays PSU's. I have an Antec Gaming True 850w PSU still running strong. Hope that answers your question sufficiently. Good luck!
Not only is the technology better but the capacitors tend to get more 'leaky' as time goes by. They don't perform as well as when new and can start having an affect on frequency timings which are a big factor in switching power supplies.
Many PSU's come with 3 or 5 or sometimes even 7 year warranties which covers them for the time frame you mentioned.
Efficiency is the main difference, 80 PLUS ratings have recently introduced a "Platinum" standard. Generally as time goes on all technology is gradually improving, the cooling system, the components and capacitors, the efficiency.....all generally getting better/higher quality as time goes on