1) The need for 4-5 PCs per group (especially with the interactions between PC classes). As I've said, my last BFRPG game, lasting from level 3 to level 7, was a blast, and had only one PC magic-User and a companion Cleric against home-brewed encounters (I don't know if they would have fared well against a module). In 4E I'll probably need more PCs around to run the game properly...
Not true PER SE.
You can run a solo game in 4e. I ran a short campaign (2 adventures) with a warlock and a ranger (both are strikers, like the rogue).
You just have to retool it to fit. For instance, a game with just a Rogue (and a companion character) would likely be more intrigue, more "Breaking and Entering" and jumping someone from the shadows. Very much like the game
Thief, or an Indiana Jones-style game. Meanwhile, a game with just a Wizard would/should look like a Western; everyone diving for cover, shooting from behind cover, the wizard battling other controller/artillery monsters. Possibly arcane duels, in addition to various research/investigations/exploration.
A game with 2 people means using 2 monsters of equal level, ONE elite monster of equal level, OR one monster of higher level (so if you have two 1st level PCs, then use a 3rd level monster). Sprinkle two minions here or there, and you have a good game.
You (as the DM) have to be aware that a PC going unconscious in a Solo/Duo game may mean that the PCs lose. Either a TPK or "you're captured".
A game with 1 or 2 PCs might mean more Exploration (like oldschool) rather than constant fighting. Just "Going and seeing what is out there" type stuff.
4) How easy is new material (such as new races and classes) to make for 4E? I love homebrewing, so I'd like to use a system that's easy to homebrew for (that's one of the things I love in Traveller).
Races, monsters, and powers are a snap to make. One of the great thigns about 4e is how easy it is to make monsters/NPCs. It's very "effect based"; "I want to make a fire monster. So I just make some effects that feel fire-y, check to make sure that's balanced, good to go".
Classes, on the other hand,
much harder to make. There's not really guidelines on how to make them, balanced wise, etc.
5) The entry cost. That is 60$-100$ for the three core-books alone, about 30$ per additional PHB or DMG, and, if I follow the suggestions given above, around 70$ per year of subscription costs to DDI. And that's not including miniatures and/or a battlemat... It adds up, and is quite a point I'll have to consider, financially speaking.
Starting in September, they are going to be releasing a line of products called "D&D Essentials", starting with an actual Red Box. The products will be much cheaper (I think like $10?). If you are willing to wait...
Also, I bet you can find used 4e books cheap. A lot of folks who didn't like them likely want to get rid of them.
Not to mention that you, the DM, are well wtihin your right to say "Hey look, you want to play something that isn't in the PHB, YOU (the player) buy the book, and you can use it." This agreement means that you the DM have to look at the info in there, so you understand it. But this would take some of the financial burden off of you.