I'm not at a MC firm, but am now 12yrs PQE in a top regional firm, and have a sister on the partnership track at a MC firm.
Just to reiterate what everyone else has said really - mistakes are acceptable, provided its not the same mistake twice. If you don't learn from your mistakes, that will go against you. Seek advice as soon as you realise you've made a mistake so it doesn't become a crisis.
Attention to detail - will really make a difference. Take notes of your instructions, check at the outset if you don't understand what you've been asked to do.
Asking questions - don't go to your solicitor / partner with a question that you don't know the answer to without having looked at trying to answer it - i.e. demonstrate that you've done some research, looked into recent authorities etc so you can show that you've at least tried.
Get on with everybody - the doorman, the secretary, the PSL, the copy room. You never know when you might need them. Be professional at all times.
Don't ar$elick - you'll make yourself unpopular and when your team is asked for their comments, no-one will be complementary if you've only made an effort with the partners.
And think carefully about the long term plans you have - law is quite odd in that although there is a HR team etc, the decision making is done by the partners / heads of team etc who are generally not management trained. Partners etc have made partner because they're very good at law, not necessarily at team management. So whilst things are changing, MC partners are still (sweeping generalisation) middle aged men. You will have to work twice as hard / give up important events sell your soul to be very successful. I do agree, even in the regions, that long periods of maternity leave, or 2 periods quite close together, or any PT working are generally fatal to making partnership. Of course there are exceptions to that, but that certainly my experience.
If you are prepared to do that, fine, choose your specialism carefully and make sure your DP (and children if necessary) are on board with that too.
If that's not your long term plan, than take everything you can out of the opportunity from a learning / client contact / development point of view, save financially as you'll probably have to take a pay cut if you leave the City.
Good luck!