Hi, she sounds very much like my 2.5yr old. I agree with sacha that letting the less serious things pass by, so she knows when you do mean business.
Things like sitting at the table, my dd normally happilly comes, so on occasions when she'd rather carry onplaying with her toys, I ask her about three times to come and sit down, then i say oh well you'll be hungry. Then me and dp sit down to our meal and make exagerrated noises about how delicious it is, she always comes to the table within a minute! If she didn't come at all, i wouldn't fuss, and if hungry later she can have some bread and butter or fruit, but definitely not another meal specially prepared.
Sharing toys is really tricky, my dd is a lovely angelic girl when alone, but when she has a playfriend round, she gets really possessive and will snatch toys, or push another child. First of all i try distracting, playing really exagerrated games of sharing, oh aren't you kind giving this to so and so etc. If it continues i take her hands and make her face me while i quietly tell her that if she doesn't play nicely she'll be on the naughty step. If she continues to snatch i start counting to three and she normaly gives a toy back, if not she spends two mins on the naughty step.
Now i've told you what to do, but this doesn't stop it happening the next time and the next time! so there must be something i'm doing wrong!!
Although i don't think its that, i think as you say, when they get older and more able and more understanding, there will hopefully become less and less of this behaviour.!!