Multiplayer is indeed the most important part, and IMO this is the biggest area where PES4 failed. There was a huge divide in PES4 between folks who liked to tinker with formations, play longballs and attempt to play smoothly and realistically, and then people who played very cynical football, jerking back and forth a few times, short passing back and forth, NEVER using the radar, rarely giving a crap about formations, until they were inside the box, pass one more time and score (because defenders were pretty useless in PES4). This was frustrating to me and reminds me of old fighting games where one guy would try to play with style, and the other would just do one cheap move over and over and end up winning.
PES5 comes along and suddenly formations matter, defensive positioning matters, playing longballs can be used to great effect, etc, the list is honestly huge. Massive improvement and suddenly now my friends have all developed their own styles: some like to play hard and fast with longballs and high counterattacks, some play very fast but very little longballing and more short passing / dribble (brazil!), some play defensive, some spend serious time building up plays... and the most important thing of all is that all of these styles can be effective! To me, this is such a huge improvement over PES4 (where the only 'style' of play was cut back, cut back, cut back, pass/pass/ cutback pass shoot).... that it's almost hard to express how much better I feel PES5 is.
For all the frustrations with PES5, i can think of wayyy more in PES4 that pissed me off to no end. I spend an hour plying about with my formation only to have a guy basically toss high through-balls to some super-high-stats striker and 80% of the time my defender stumbles for about 2 seconds and lets him through for a 1-1... awesome..
PES4 reminds me of NBA Jam..