I'm loving Borderlands so far, but it does run into a problem that affects all co-op RPGs. Each player must pause from time to time to select a new skill or compare items in inventory and switch equipment. The problem is that all the players in a group don't share that pause. One player is kept waiting while another makes choices.
One player levels up before or after another player. Because they don't level simultaneously, one player is waiting patiently (or impatiently) while the other looking at skills and considering which to select next.
One player loots a weapon that compares favorably to something already equipped. The other plays waits patiently (or impatiently) while he or she is comparing items and deciding what, if anything, to switch.
How big a problem this is depends largely on the personalities of individual players and the degree of familiarity between them. Some players think nothing of it because they're used to it. But it generally is a barrier to fun.
So what are some possible solutions?
One option, which I seem to remember experiencing once or twice before (in Gauntlet: Seven Sorrows?), is to set intervals at which all players receive their rewards (new levels, skills, and such) simultaneously. An advantage of such a system is that developers can keep the action flowing and the experience unbroken until a bit of respite would be ideal for players. Another advantage is that players do not feel pressured while making their skills/equipment selections and are likely to discuss their options with their fellow players.
A second option is to pool the experience points and loot in such a way that co-op players necessarily reach their goals (such as a new level) simultaneously. I'm not sure how most players would react to such a system in regard to leveling, but achieving this would a loot system would certainly be difficult.