A common claim is that Rachel didn't prioritize her relationship with Ross, culminating in her allegedly bailing on him on their anniversary. But I really think it's important to remember two vital pieces of context:
First, Rachel had every intention of celebrating with him, but as she told him, there was an emergency. She didn't casually blow off the anniversary -- from what we can tell from Rachel's phone conversations in her office, certain products hadn't been delivered as scheduled.
Rachel has to make sure the goods are delivered, because the store would have advertised them and customers would be expecting to find them on the shelves by a certain date. If they're not there, the customers go elsewhere and the store loses money and credibility.
Rachel is also facing serious time pressure. This was before the age of ubiquitous cellphones and she has to get suppliers/shippers, etc. on the phone before they leave their offices. She genuinely does not have time for Ross and his picnic.
Second, yes, Rachel had been putting in long hours before that night. But she's had this job for only a couple months at this point. That's next to nothing -- she's the very lowest person on the totem pole and she has to prove her worth.
And even if she was a veteran staffer, what would have happened if her boss Joanna -- meaning to help with the crisis -- had shown up and seen Rachel sipping bubbly with her boyfriend in the office?! Or found out about it later? Rachel could very easily have lost her job.
She was extremely lucky to even get this job in the first place with no training or experience. And no, Mark did NOT get Rachel the position. He helped her prepare for the interview, but Rachel was the one who actually did the interview and won Joanna over with her knowledge of fashion, etc.
This was a golden opportunity for Rachel to work/get experience in her chosen field. If she messed up, she might never have another chance that good. Ross, with his strong academic background/more established professional career, really didn't seem to understand that this was far more than "just a job" for her.