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...
As others have mentioned, 4E actually scales to a lower number of PCs reasonably well. The xp budget rules remain pretty easy to use, and you actually have decent options for what to present the PCs with - one or two average monsters like normal, or several enemies in the form of minions or lower-level enemies, or one scary big bad guy with an Elite.
2) The limits on wizards, even though Rituals probably mitigate this a bit. I simply love DMing a party with a wizard in earlier editions (or BFRPG), especially when the players are being creative... So many possibilities for a crafty player to use!
There definitely are limitations in place. Still, Rituals plus Utility powers plus Cantrips (Mage Hand, Ghost Sound and Prestidigitation At-Will), plus clever use of skills and random gimmicky magic items, can leave plenty of options open for crafty players!
3) The limited scope of the main PHB. If I'd want Druids, for example, I'll have to buy yet another book. This makes the entry cost a bit high...
Yeah, it basically traded out some older options for newer ones. Still, one month of DDI gets you pretty much everything you could need - and WotC does offer occasional deals, like the bundle that included both PHB1 and PHB2. That can cut down on costs quite a bit.
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).
Remarkably easy, actually, since the formulas for system design are often pretty open and easy to balance against. Races are an absolute snap to create, and same with Paragon Paths and magic items. Monsters are easy to design for as well. Classes are a bit more challenging, mainly because of how much goes into them - both the base class features and 30 levels of powers. On the other hand, adding new builds for an existing class isn't too hard, so that can be an easy approach to customizing things for your personal setting.
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.
Like I said above, bundles, and DDI, are your friend. One month of subscription makes an excellent starting point - if nothing else, it can tell you what products you actually are interested in, and you can focus on the material there. Maybe get the group to split the costs on a battlemap, use tokens or the like instead of minis, and the investment shouldn't be all that bad.