Multi-classing has always been a problem for D&D. The 1e system was horrible and stupid, and the 2e system was not much better. In 3e you had some combinations that led to serious deficiencies (especially for spellcasting classes) if you made certain choices on the one hand, but also had the potential to become ridiculously broken in the hands of a savvy player. It was also really complicated, with a lot of room for customization (especially when you started sprinkling in prestige classes) but also a lot more work. It looks like in 4e they tried to step away from the concept entirely and focus on the "iconic" classes that have been there since the beginning. As I've mentioned, I think that this is a shame, especially since from the beginning of sword and sorcery fantasy there have been characters that straddle the line between two "classes." But, simply from a design perspective, I think it works for the system, and it probably avoids a lot of headaches that might come from cherry-picking amongst a huge pool of powers.
I always wanted to try one of the games out there that do not have a class system. For example, there's a superhero game called Mutants & Masterminds that simply assigns every character a number of "power points" per level. The player can then purchase his stuff in any way he sees fit. So a +1 attack bonus would be 3 points, +1 to Defense (equivalent to AC) is 2 points, 4 skill points cost 1 power point, a feat costs 2 points, etc. The bulk of these you wind up spending on special superhero powers, which are analogous to various D&D class features. It was an interesting system that I had wanted to explore but never got the chance because the planned game fell through. Anyway. I hope you have enjoyed the preceding random digression. The end.