[nolemmings]

Well I'm sure you know the main factors, but here is what I believe is typically considered.

1. Color vs. B&W-- some greatly prefer one over the other and in pre-war that translates to lithograph vs. photograph, as there really weren't color photos then.
2. Size of set-- some focus on smaller sets that are generally easier to complete, while others like the larger set as being more representative of the year/era.
3. Availability--some are far more patient than others. Announcing you are going to put together an e107 or Just So set is great but it almost certainly will never happen. Some don't mind waiting months before adding a card or two to their set while others need a fix far more often. For this reason many chase more than one set at a time.
4. Condition--some must have a certain condition met for their cards that is next to impossible to satisfy in some sets.
5. Your favorite team/where you grew up--this is highly important to some.
6. Price--again, some may just be unrealistically expensive, or at a price that you would prefer to complete two or more different cheaper sets instead.
7. Investment return--motivates some, not me, although no one wants to be too speculative and take a bath on resale down the road.
8. Overall attractiveness-- beauty is in the eye of the beholder.
9. Player selection-- some sets are gorgeous but contain only or mostly players whose names are known only to the harder core baseball fan. Others are more mainstream but still miss some major stars of the year/era for some reason.
**10 (added). Portrait vs. Action. Some prefer to see depictions of the player's faces to them in some less-defined action shot. Others vice-versa.

These are not listed in any particular order and there are others that will be mentioned no doubt.

Personally, I like the black and white sets, or black and white with colored backgrounds or borders. Nothing too easy, nothing too hard. If you told me it will take me 10-15 years to complete a set with cards of the condition I like (mid-grade or better) on my budget, I wouldn't flinch. I will say, as will many others here, that several card set projects have been started that are jettisoned in favor of something else--there's nothing wrong with that. BTW, there's nothing wrong with type collecting either-- I like it actually, as you do not feel enslaved to a particular set and the sometimes ridiculous price demands it places on what seem like common cards.