Most of my last three hours with Path of Exile has been spent getting to grips with the character progression mechanics. Not directly, mind, because gaining levels is still something that takes significant amounts of time, but I've been playing around with alternate characters and advancing a couple of levels, just to see what my options are.
I think I've got the hang of it. All the character classes are notionally identical, except they start at different parts of the same huge, sprawling skill tree. You gain skill points as you rise in level, and theoretically two different character classes could build towards each other, eventually overlapping in capabilities and being more or less identical.
The trick is that all of your character's active abilities (fireballs, double-strike sword techniques, etc) come not from their class, but from magical gems that slot into their equipment. The abilities they learn from leveling up then make certain classes of equipment more powerful. Thus, if you build your fighter (marauder) as a mage (witch), you can slot as many spells as you like, but since all the nearby nodes of the skill tree enhance physical attacks, you'll be basically wasting your levels. So, if your plan is to bridge the classes, you have two choices - be prepared to accept that you're suboptimal, or play as a marauder for the 20-30 levels necessary to get to that part of the tree and simply bank the best of your witch weapons until that happens.
Although, maybe that's not such a bad plan after all, given the inevitable gear turnover that happens in these sorts of games. Since, as a witch, you'd have to replace your wand every 3-4 levels anyway, maybe junking all the witch gear you find until you're ready to make the transition is your best bet. My only worry with such an approach is that you might get used to playing in the marauder style, and thus have a painful period of relearning the game's mechanics right around the time most builds start to get powerful.
Also, you'd dilute your character's abilities and have a lot of superfluous strength abilities that you might not need as a magic character. From what I can tell, you might be able to mitigate that with respeccing, but the process is so byzantine and time-consuming that the only reason you'd do it is if you absolutely wanted to use the marauder's character model (a big, beefy dude) instead of the witch's (a waifish looking woman).
Which, now that I mention it, does seem a little odd that they would tie the character models to specific classes. I know a lot of people have some very strong preferences about how their character looks (especially when it comes to gender), and were I one of them, I'd have to count this mandatory appearance/ability connection as a weakness of the game.
Still, a lot of the stuff in the player shop is cosmetic, so next time I have a few idle minutes, I'll have to check and see how much you can customize a character purely with vanity purchases.