There is a current of opinion (for example) basically saying that the London attack was crap because it killed very few people. That's showing admirable resilience, and it's the right attitude to have; but as far as analysis goes, it misses the point.

The 9/11 attacks were also really bad as far as killing is concerned (the towers were practically empty, it could have been far far worse during the day), and this is naturally the aspect we focus on emotionally: those deaths are tragic. But the intended effect is much more political, emotional, economic etc. The humiliating aspect it had is also rarely discussed, for obvious reasons.
On the whole, it did what no ground attack by an "Al Qaeda army" could have ever dreamed to do. If the intention was just to kill a lot of Americans, then they could have proceeded otherwise with much more "success".

It's the same for the London attacks. It's not the deaths that count strategically, but the fear, acrimony, and political ends created.