1) Sticking Gap on everything. That kind-of logo/advertisement in huge lettering is great on the way up, but not on the way down. Superdry take note. You could not pay me to wear either.
2) Pricing and price point. It's frankly been bizarre for years. Overpriced when compared to new competitors, such as Uniqlo. Not quite good enough to compete with other stores, such as Levi's or more designer brands. Its position in the market felt awkward in recent times.
I still buy chinos and plain t-shirts from Gap. No logo and so nobody knows. Despite their crazy vanity sizing, they still offer a range of sizes unlike Uniqlo, which only offers tailoring in-store at present in the UK.