Mediocre HR. The vast majority of the people do not really know what is it that they are doing. I have been working for the company for the last 16 years and I have seen how a great company goes down as one of the less exciting places to be on earth.
Innovators, entrepeneurs, knowledge seekers are going to be swallowed by a very conservative corporate culture where guys that do not even know how to turn off an iphone are managing IT departmens or innovation innitiatives.
For new joiners:
They will tell you there are overseas opportunities, there are none. I am a senior manager in the company and even at that level, expat positions (or short term international assignments "STIA" are almost non-existent).
For experienced hires:
You will join with lots of promises being made, but reality is that you will be stuck in the same "job group" for 4 - 5 years when the average Shell employee is moving up the ladder every 2- 3 years. Once you are due for a promotion, they will trigger a "re-organization" and you will recieve a redundancy letter that will (mentally) force you to say yes to any internal offer they will throw at you. Be aware of HR. Really.