Kv, how would they reconcile the fact that I've got an essentially maxed character and my housemate does not? Will the game be easy for me and hard for him just because I put in another five or ten hours of play? It makes more sense to me to have a new character for a new game, and I think I'd enjoy that more.
Also, I have only ever seen it said that 'choices made will continue to have an effect', not that you will get to play your character still. Where is there room for expansion and development in your character now?
That is just the way I see it.
I assume that Bioware will use the same tricks it used in Baldur's Gate 2, that most RPGs have utilized in the past. They set a minimum level of experience for ME2, say, level 50 or 55. Like in most games with a level system the higher you go with it, the less impact a new level has (although in relative terms, ME's leveling system was very, very gradual anyway)
If you saved a character from BG1 and ported it to BG2, it would measure your xp level. If it was below the minimum, it would automatically level you up to that point. If it was above the minimum, then your character was unaffected. That way you don't feel cheated if you went through and did absolutely everything in BG1. If you started a new character in BG2 you were set to the minimum. Another thing, Bioware games and western RPGs in general are very flexible as a general rule - I know of a few eastern RPGs in which you cannot proceed through the game unless you're at a certain level. In games like BG2 and ME they have a system under the hood that adjusts most encounters somewhat based on your character ability (though not to the Oblivion extreme)
Another near-inevitability is that they'll overhaul the character creation system for the higher levels. Using Baldur's Gate as an example again (because it's such a good one), in BG2 when you created a character you had to choose your weapon proficiencies, and there were quite a lot of them. Long swords, scimitars, two-handed swords, katanas, daggers, short swords, etc. But if you started a character in BG1, you'd find that there were only over-arching categories of weapons. Long swords, scimitars, et al would all be encompassed under the Great Sword proficiencies, daggers and short swords would fall under Short Blade proficiencies, and so on. If they had kept BG1's proficiency system in place for BG2 a mid-level warrior would competently be able to use every weapon in the game, which is broken. So they shook up their system. In ME2 they'll probably increase the number of weapons you can use and add more passive skills (like fitness)
I think it's likely that they'll completely overhaul the system. If you had a mage character from BG1 and you ported it to BG2 you'd have to level up your character from scratch no matter your circumstances. They had added and removed a number of spells from the game, so on the off chance that your character had learned a spell that was no longer in the game they let you choose your spellbook again from the new list. The downside to this was that if you had a full spellbook at the end of BG1 you lost a good number of spells. ME2 won't have that problem because you don't gain abilities outside of leveling. But they could very well change things up from ME's rather boring "progress bar" skill progression to something more interesting, maybe something that differentiates different characters of the same class. Perhaps a skill tree w/ stats ala Diablo 2. Really, utilizing one special ability per skill bar was not a very smart move on Bioware's part. If they do change things up in ME2 they'll have to make every single player reroll his character but I don't think that will be a big deal considering how relatively low-hassle it was to level up in ME1. Overhauling the system will also allow them to rebalance the game and make Shepard not quite a God without actually having to explicitly reduce his power.
Continuing the whole BG2 thing (they've said that BG2 is their template for ME2, which is encouraging) I would also expect that some of the party NPCs from ME1 are killed off in ME2. Only a few of BG1's many potential CNPCs made it into BG2. A few were killed off in the time between BG1 and BG2. Whatever event leads to the teaser I would expect would kill a few NPCs. My money's on Tali - relative to other characters she got pretty short shrift with regard to backstory in ME1. Several other characters could bite it. They'll fill their shoes with new NPCs, and they may expand the pool beyond the standard ME1 "one character of every character type" template. Maybe they'll add some NPCs who are less benevolent than "mercenary with a heart of gold".