When I first heard about Rogue Trader's Career system, I wasn't massively impressed.
I had seen the glut of Class-based systems, and wanted something a bit different. While I had heard of WFRP's Careers, wide and varied as they are, the whole "buying Advances, not automatically gaining new stuff" was a foreign concept to me.
For those of you who don't know: in your average Class-based game (like D&D), you gain Experience Points - once you reach a certain threshold, you gain a new Level. With that Level, you automatically gain new abilities (like new spells for Wizards, or Wildshape for Druids, or extra Sneak Attack Dice for Rogues, etc). While you can make some choices (like what spells to pick, or Feats in 3rd/4th Edition), your advancement is pretty linear. It leads to a "video-gamey" feel, in my opinion.
WFRP and the 40RP system, however, work by Rank and Advances.
So, you're Rank 1 - the lowest of the low. You have a list of Advances (whether new Talents, bonuses to Skills, whatever), and their XP costs. As you earn XP, you can "buy" these Advances - and you go up to the next Rank when you've spent a certain amount of XP, not earned it. To me, that makes it feel like more of an accomplishment - you made your character better, instead of watching them get better. It also allows you to buy any skill you want - at an extra cost. Such a system allows for a truly diverse set of characters, even using the same Careers.
And, the RT Careers are pretty cool. The Rogue Trader is aimed at the swashbuckling, order-screaming Captain, while the Arch-Militant is Death Incarnate (with some extra skills to fill out some other roles too), but none of them are as cool as the Seneschal.
A Ninja Spymaster Accountant.
Rogue Traders generally end up with both more money and more possibilities than they know what to do with. The Seneschal is how the Rogue Trader deals with such things. A master of both trade and intelligence, the Seneschal acts to further the profits of his Dynasty. Whether it's collecting information on where they will get the best deal for their goods, being knowledgeable about the other Rogue Traders operating in the area (and whether or not they might be open to a temporary alliance, or at least parley), or knowing the actions and locations of the rest of the crew on board, the Seneschal will easily secure himself a great position of trust and power aboard the ship - mainly, because they have enough information to make any ignorant Lord-Captain's life a living Hell.
The Seneschal has something of a focus on Fellowship and Intelligence-based Skills - like Commerce, Barter, Charm, etc. They have a special ability to pass any Intelligence-based test once per session by spending a Fate Point - making them great at their job.
They also get quite a few good combat skills, mainly in the vein of being faster and shootier than most. All in, it gives them some of the flavour of the traditional "Fantasy Thief"-type classes, but with an extra element of intelligence-gathering and bartering out the wazoo.
It's one of my favourite Careers in the 40k RP books to date!