I've been thinking about the ME1/ME2 discrepancy for a while now, especially playing the firs two games consecutively.
The fact was, ME1's shooter gameplay was pretty bad. Unintuitive cover system, annoying "Overheat" system, and no reward for accuracy/aim. ME2 was just a development of that, making the shooter gameplay a lot better. Noticeably the powers are much more streamlined and "fun" to use too.
This was coupled with the removal of the open planet exploration. Personally this wasn't an issue for me, as roughly 75% of planets in ME1 couldn't be explored anyway, and the "planet" was usually a cookie cutter barren landscape that looked suspiciously similar to the last planet you visited. Scattered around this barren landscape were a few minerals to scan, or a probe to salvage, and then usually your main objective.
Looking retrospectively, ME1 gave the illusion of free roaming, without actually making it that rewarding. In short, ME2 streamlined and cut away the parts of ME1 that were less interesting and relevant. The sidequests are still all there; and relate more to the most important aspect of ME2, your team. The game is also still very much an RPG, but in a less conventional sense. In ME1 you got XP for everything, which didn't make a whole lot of sense. Simply looking out the Citadel window, for example.
Many people lament the loss of ME1's inventory and squad upgrade system, but I myself found these very cluttered and confusing to someone trying to access the series. There were simply far too many things to upgrade; it was overwhelming. Likewise, with the inventory, There were a multitude of armour sets, weapons, and biotic amps, with confusingly similar named (i.e. Scorpion I-X) items. More user friendly does not = dumbed down. The ME2 system of upgrading your powers, switching your weapons, and leaving armour customisation to Shepard alone is infinitely preferable, I find.
And, at it's key, Mass Effect is clearly a game about the story. And ME2 doesn't dissapoint. You can find fault with the squadmates, their stories/loyalty, and the overall format of the suicide mission, but for a game about great stakes and the potential destruction of the galaxy, I thought the chaos and the preparations you had to make to ensure your teammates didn't die was a complete success. And particularly in that part of the game, the voice acting/scriptwork/soundtrack made my hair stand on end. Incredible.
I love both games, but if you think ME2 is anything but a development of what was already there, I think you're playing the wrong series/looking at the original with rose tinted glasses.