There’s a little bit about how the furniture of the Tabernacle is positioned during Eli’s time that gives us an extra sense of the problem Eli was as a priest.

3 And ere the lamp of God went out in the temple of the Lord, where the ark of God was, and Samuel was laid down to sleep;

4 That the Lord called Samuel: and he answered, Here am I. (1 Samuel 3:3-4)

The lamp of God is the seven-branched candlestick, and it was supposed to never be allowed to go out!  Yet we are told here about stuff that happened before the lamp went out as if it were a common occurrence for it to be allowed to go out.

It may be that Samuel was assigned to sleep there and light the lamp first thing in the morning in order to make it look like it had never gone out at all, in which case Eli was trying to conserve oil in a place where it was not meant to be conserved.

Also, notice that the lamp was where the ark of God was, and Samuel was sleeping there too!  The way Moses instructed Israel, the ark was to be in the holy of holies and the lamp was to be in the holy place, separated from the holy of holies by the veil.  But here, the lamp and the ark were together!  We don’t know whether that means they were both in the holy place or both in the holy of holies, but we should know that whichever it was, it was wrong. 

So Eli had moved things around such that the ark was commonly accessible. 

This is quite shocking.  The ark was supposed to be in the holy of holies and only be visited once a year on the day of atonement by the high priest, who would sprinkle sacrificial blood on the mercy seat.

So it seems that not only was Eli allowing his sons to take the fat of the sacrifices and not only was he not rebuking them for their immorality, but he had moved the Tabernacle furniture around in such a way as to effectively change the ordinances, he was neglecting the care of the lamp of God, he was making the ark commonly accessible, and he was making Samuel treat the holy place like a bedroom instead of a temple.