BL2 offers four character classes, who use the same gear but have some different special abilities. I'm playing a Commando, which seems like a solid beginner class due to a robotic turret that adds to my DPS, picks off flying mobs that are hard to aim at, and off-tanks by pulling enemies for me. The other three classes include a "Gunzerker" who can dual wield rifles, a robot stealth assassin with a focus on melee combat, and a Siren psychic character who seems to focus more on crowd control and support. (The latter is NOT the subject of the notorious "girlfriend mode" comment made by a developer a few months ago - that is a new mech pet class slated for DLC - but it may be worth noting that the one existing female character seems the most focused on support.)
In terms of incentives, people make a lot of the loot, because there are so many varieties of weapon, but this part doesn't really stand out to me. There are tons of random properties, but many of them are useless. For example, I found a shotgun that takes 3 rounds per shot and had a clip bonus allowing it to carry a total of 11 rounds - i.e. still one shy of being able to shoot four times between reloading, so it could have just as well been a -1 clip penalty for all the benefit it added. Perhaps if I play through more than once I will try a different playstyle, but on my first character it seems that really 2-3 stats matter the most, and I will only consider gear that upgrades those stats.
Overall, I'm not seeing where the replay value is going to be in this game. I had a really rough time for the first few quests, as the story very quickly dumps you into level 5 content at around level 3-4, but once I survived a few side quests and obtained more optimal weapons things started to fall into place very quickly. By the time I finished the newbie area, I was level 8 and frequently one-shotting the level 5 content. I could see challenging yourself by trying to finish just the story quests with as few side missions as possible in order to limit your access to loot and exp, but that approach comes at the expense of missing most of the content. That said, the ability to out-level content for an easier path is probably a big part of why I'm enjoying a genre that is usually not my favorite. If I do end up sniping mobs one by one through the entire storyline, BL2 will be more of a success than many games I've played.
Business model aside: There are announced plans for four DLC packs (one of which was free to paid pre-orders - not sure if I will get this with my free copy, but that's a small price if I decide I want it compared to a free game), with an already announced bundle to buy all four for $30 (versus $10 each). I guess it's an odd quirk of the business that you want to get your DLC out there early and paid up front; your theoretical market for DLC increases as more total copies are sold, but it's harder to capitalize as more players finish or quit the game and aren't as interested in more DLC. Sadly, this is what Bioware was getting at with their comments on how those of us who don't like day-one DLC are out of luck.