I actually wouldn't like the mulligan, since it would completely change the deck building aspect of the game IMO. If you only need 1 of a single card but you need it early you may pack as many as 3-4 in you deck to ensure you draw one early. However, with a mulligan, you would only need to pack 1-3. This frees up 1-3 more spots in your deck for other cards.
Having a mulligan feature would also make the contrast between good and bad decks smaller. For instance, if I have a rather bad deck because it's big and bulky and has no extra drawing, then it would normally be considered a bad deck due to statistics. While a similar 30 card deck would still be better even with the mulligan, the worse (and bigger) deck would perform much better with a mulligan.
Think of it like this:
Without Mulligan:
60 card deck: 3/10 rating.
30 card deck: 7/10 rating.
With Mulligan:
60 card deck: 6/10 rating.
30 card deck: 8/10 rating.
This would be because the 30 card decks are built to not [bneed[/b] mulligans. You would be making the gap between skilled veterans and newbies much smaller, increasing the luck factor for games between these two groups. But I'll admit games between skilled vets and games between newbs WOULD rely less on luck.
Anyway, getting off of my soapbox, I voted for the mark change, new cards, and more mid level PvE content.