I haven't played Dungeon Siege 1 and 2 myself, but I guess I would be pissed if I bought a sequel and found out that that sequel has very different mechanics and different emphasis. Some players who prefer less story-oriented hack and slash (like D2) will find this off-putting. Although to be fair, I believe there was a demo - so instead of having absolute faith in the franchise (despite a change of hands), one would have largely avoided the issue altogether.
Having 9 skills can be a bit boring in the long run though. Yes, there are various ways to customize it via proficiency (is it proficiency?) and various strategies employed with it. But there is only so much to cycle through and so much to see. Not sure how "simple" the game is - Normal was quite manageable without thinking too much about the build - but there are underlying mechanisms which might be worth thinking about in hardcore maybe? For example, choosing gears, how to build up your skills/passive bonuses, combination of skills vs certain enemies, chaos effects - they all can lead to interesting combinations.
Gears could definitely use more depth though. Not sure what the sentiment regarding D2 is, but I found the system to be highly gratifying - set items, socketing, Horadric cube, charms, unique properties of some items (skill increase, skill use otherwise not accessible to a character, release charged bolts upon getting hit). This is one of the main reasons why D2 has massive amount of replayability (to some of us). Though admittedly Blizzard spent long years honing the system.
I must admit that the independence from stocking up prior to a battle - buying a crapload of potions, collecting a huge amount of charms, town portals - quite refreshing.