*Membership spots not really limited!
Loss of self and personality?
It's why I loved Tennant's regeneration. I think it's the first time we saw how important each Doctor's personality was. They're all the same person with the same memories, but they're also not the same person in some ways. I loved that "I don't want to go" line..
The energy--whatever it is--that allows the Time Lords to regenerate is limited. When the Doctor, Rose, and Mickey got stuck in an alternative London and the Tardis was essentially dead, the Doctor used some of his energy to, basically, jump start its battery. He said he's given up a part of his life in order to do so. When River kills the Doctor, she gives up the entirety of her regenerative abilities to bring him back. Old series or new series, there is a limit.