Well I would say salary amnd start date are definatly the main things. Some people will want to know other things - how many days holiday they get; if private health care and life assurance is included; what the pension scheme is like? - how much the company will contribute, how much do they need to contribute; if there is a bonus scheme (probably not that likely anywhere at the moment!); if any interest free loans are offered; if season ticket loans (for your travel) are offered; how many times the company will pay for your exams (standard is that the company will pay for the first sitting, and then any resits must be paid for by yourself); how many times you can resit an exam (most companies only allow resits if you gained a certain percentage the time before, if you failed really badly you can be straight out!); if you are tied to the company for a certain period of time after you are qualified (if you leave they require yo to pay back your training fees); and additional things like gym membership, etc!!
However, these are all normally set by the company. They either offer it, or they dont. If you get an offer with a top 4 they will include loads, with a top 10 they will also include most things, and with a top 30 you should still get a great offer.
At the end of the day you will be getting a great qualification, with a good company. I would recommend that although comparing the benefits between offers is good if there is a huge difference, if you really like the company and the pay is enough, then the rest is up to personal preference of what you feel you need!