I will eventually end up getting the DS campaign guide simply for the themes in it. I agree that themes are an excellent addition to the game and a great way for making different characters stand out from each other. (i.e. a great way to make two different two handed weapon fighters feel different from one another).
I agree that I am not certain how themes could have helped with the math fix, so I would be interested to hear your thoughts on that. Not saying it isn't true, just that it isn't readily apparent to my overly tired brain (stupid 12 hour days).
As for multiclassing, I really like your idea of themed multiclassing. Multiclassing in 4ed is just . . . bleh. I suppose its pretty cool for bards, but even then the feat Bard of All Trades took away most of the biggest benefits of multi-classing.
In sum, I personally am a big fan of customization and I think that themes potentially provide a great method of achieving customization without really adding to bloat. More classes does not per se equate to customization. More feats doesn't either, especially since there are so many "sub par" feats out there and the sub par feats tend to be the ones that best exemplify customization ("I'm really knowledgeable at History" frex). Themes allow us to customize, to create different flavors, and to do so without really adding very many rules to the game.
The added rules I think is the biggest problem with new classes in particular. One of my players just rolled up a Sentinel for his 11th level character that was being added to the game after another player had to drop. Unfortunately, he didn't have the actual book and didn't think to read the class description in the CB all that carefully. At the first session he was looking at his power cards and pulled out the "Combined Attack" card which is an encounter but showed four little check boxes on the bottom. Unfortunately I was not familiar with the Sentinel either in part because I was not entirely certain what class he was playing until the night before. We were confused by the card and assumed it was a CB "bug", and so ruled it could only be used once. Of course, I also was not aware at the time that he only had one at-will (which knowledge may have lead me to a different conclusion).
Fortunately, after that first session we both looked it up and realized that yes, he did indeed get 4 uses of it at 11th level, but the point is that its almost impossible to keep up with all of the rules when they keep pumping out more. I consider myself fairly knowledgeable about the rules, but I don't have time to scour the rule books cover to cover and memorize every possible interaction with every possible class etc. I try to familiarize myself with those rules that I know will come into play in the next session or two, but you cannot always predict everything that will happen in a given session. Its this bloat of rules that leads so many DMs to restrict sources, which then often leads to player discontent because they cannot play the character that they want to play.
