The rats are nicely atmospheric. This is the first area of the keep that isn't lit by goblin torches; players have only as much illumination as they have brought with them. The rats hide amongst debris and stalagmites, scuttle across the ceiling, and swarm players who become isolated from their allies. They're minions, though, so every player is likely to kill at least one a turn, and rangers, fighters and wizards will probably slay two or more.
The weakness of the rats is supposed to be rounded out by an Ochre Jelly, which is a fairly credible threat to a second-level party. The Jelly moves very slowly though, so if the encounter triggers early the rats will probably all be dead before the Jelly shows up. In the game I ran, I started the battle while the players were in the north-east area of the caves; in retrospect, I should have waited until they were almost on top of the Jelly before revealing the rats. (There's no indication in the module that this is a good plan, and in fact the starting rat layout seems to specifically contradict it.)
The Jelly is an interesting creature. It has 102 HP and the following ability:
* Split (immediate reaction, when first bloodied; encounter)I took this to mean that each of the halves was no longer bloodied and had the encounter power refreshed; therefore my Ochre Jelly kept dividing again and again. The players seemed to really enjoy this rather brutal interpretation, so in this instance it was a success, but it became clear that in order for second level players to win against this mechanic the Jelly would have to split into no less than 16 component parts (possibly more) all hitting as hard as the original. That can't have been what was intended so I'm guessing the Ochre Jelly halves do not get their Split power refreshed.
The ochre jelly splits into two, each with a number of hit points equal to the ochre jelly's current hit points. Effects applied to the original ochre jelly do not apply to the second one. An ochre jelly can't split if it is reduced to 0 hit points by the attack that bloodied it.
The last element to note is that the centre of this map contains a secret room, designed as a hidey-hole for Balgron the Fat. This is established as the location Balgron flees to if he escapes the Chieftain's Lair, although how he avoids the rats and the jelly is a mystery. The module has him rather unexcitingly remain hiding here until the PCs leave, whereupon he rallies the remainder of his goblins and sets an ambush for them in the Goblin Guard Room.
This is a stupefyingly bad plan, given that there are only 24 goblins in the dungeon, of whom all but three must be dead for Balgron to be here in the first place. (If the PCs took the secret tunnel to Balgron's chamber, he has no way to flee, so they must have come through the Guard Room, the Torture Chamber, and Balgron's guards.) The Goblin Guard Room is also a terrible place to set an ambush, given that the PCs will already know about the room's sole trap and it is the only goblin area with absolutely no cover or defensive emplacements. DMs taking this route can expect a weak and unexciting death for Balgron.
A much better solution is to have Balgron emerge from his secret chamber and wade into battle at the height of the player's fight against the rats and the jelly. It gives Balgron his best chance to do some damage, and to some extent compensates for the positioning problems of the other monsters in this area.
 Balgron's loot is in a chest in his room back at the Chieftain's Lair, rather than either (a) on his body, or (b) in his secret hideaway. As a result, killing Balgron doesn't feel particularly rewarding, as the players receive their treasure prior to earning it. Was this a deliberate decision?