My two cents:
For starters, the new passing system is absolutely brilliant! It's one of those remarkable innovations that make you wonder how you ever managed to play football games before it. It simply sets a gold standard for the genre.
Second, the fouls are very much improved this year and now feel logical and tied to the actual tackle. Yellow and red cards also make sense (unlike PES 9-10) and penalties are called fairly. With respect to the latter I like how refs are more cautious about calling them. e.g. a strong jostle that the ref would have immediately called outside the box would rarely be called inside the box, which what most refs would do in real life. There is of course space for improvement here: There is no reason for us to watch every single foul in replay, just the serious ones. Advantage rules need to be implemented better as well as the ability to collect the ball and quickly execute the free kick, even if the ball rolls away.
The new fouling system in combination with the improvements to jostling means that we can now control the pace of the game in midfield. Making a foul to stop the break or covering the ball in offence to seek a foul against us.
Now to the negative. There are mainly three serious things I don't like with PES 2011. In no particular order:
Attacking player returns. Imagine the following scenario: You cross into the box, the defender clears the ball away, but it ends up in the feet of one of your players in the flanks. You seek to make another cross immediately, but you can't... As soon as the ball changed possession, the defenders, rightly, advanced up-field as quickly as possible to relieve pressure in the box and/or prepare for a possible break. However, your own attacking players will only move out the opponents box at a leisurely pace, so by the time you try to make the second cross 2 or 3 of them will be offside... What's more, if after clearing the ball the opponents go on the break half your team will, for the same reason, take forever to return to their own half...
Loose balls. That is another serious issue. When the ball is running loose the AI controlled players of your own team will do nothing to intercept it on their own. The only way to get them to move at the ball is to take control of the nearest player manually, which means that, assuming you are given control of the right player on the first try, your timing will be a bit off. This is crucial everywhere, but more so inside the box, where you can see defenders gleefully ignoring loose balls, while AI team strikers run towards them like bloodhounds. Your players need to be a lot more aggressive towards intercepting loose balls.
These two problems existed in PES 9 and 10, but the last one is new or at least much worse this year.
Goalkeepers. My biggest gripe for this year. Spoken from the perspective of an amateur goalie, GKs are absolutely awful in almost every respect! I can live with the sub-par, compared to the outfield players, animations (while FIFA has amazing goalie animations), but there are certain things in their behaviour that you wouldn't even expect to see in weekend 7-a-side team goalies... For one their positions are frequently wrong. They will blunder too close to one post, leaving the goal exposed. For another their coming of the line, while timed correctly and decisive is sometimes lacking in the dive. They try to dive left or right in situations where they should have instead just made themselves "big" by thrusting hands and feet in all directions.
However, both of these are not particularly frequent and therefore not that serious. What is serious are the deflections which are nothing short of abominable! Simply put, the goalie always attempts to deflect the ball out of play or to the flanks. Never in front of him or inside the box, which is precisely what PES keepers will do 80% of the time. When you see your goalie deflect a ball you know with mathematical certainty that there is going to be a follow up and coupled with the defender's tardiness in intercepting loose balls, it is likely that a striker will get to it first, leaving him with a shot at an open goal...
Deflections such as these would be embarrassing for amateur goalkeepers. When a pro does something of the kind he would attract a lot of negative attention very quickly...
Anyway, just my two -not so short- cents.
Overall, the positives outweigh the negatives and I expect PES 2011 to be a much more enjoyable experience this year.