Overall, BJ's is just not a class act, when compared with Costco.
Before I terminated my BJ's membership, every time I walked past their tire shop, I couldn't help but notice that it looked like it hadn't been cleaned in a few decades. If a shop can't be bothered to keep its work area clean, that also tells me something about their attention to detail with their work, as well as the pride that they take in their work. And, as Mike noted, things definitely changed for the worse when BJ's tire operation was sublet/outsourced/sold to Monro.
In addition, when I did price comparisons on groceries and electronics, BJ's invariably came out higher-priced than Costco. The only advantage that I saw at BJ's was that they had a much larger selection of imported cheeses than Costco. However, that one advantage was not enough to retain me as a BJ's member.