Regarding bids...

In the early games, soon after being taught this magnificent game, players tend to be very liberal with their bids. They tend to think all trumps and any card above a 9 is a winner. As a result, they bid high and don't make.

After a few more games (or a few more hands, depending on how quickly they understand), they learn their lesson of overbidding and begin to be extremely conservative with their bids. They believe all of their cards are losers and that they can almost always bid 0 and get it. As a result, they bid low and don't make.

Finally, the players have finally fully developed. They now know that their Wizard isn't always a winner, their ace can be trumped by a 2, and that their 5 of non-trump can be a winner. Their scars have healed and now they understand that anything can win and anything can lose. They are well rounded and their various high and low bids come into place. Beware of these players.

Now of course there are other factors that come into play (simply not card players, don't have the ability to remember most of the thrown cards, only focus on making one's bids, aren't competitive, etc.), but these were the major differences I have come to notice.