In my opinion they aren't too conservative, if anything it's us (generally speaking) the gamers who are just impatient, many developers out there should study the Blizzard's policies and business model for their own success, other companies like EA and UbiSoft simply want things done way too fast, in their heads (EA's, Ubi's, and others similar) they think that they can make money by pouring out many games done fast in a short period of time, and it does work (sadly), it works because gamers continue to support that kind of business model (I'd like to see a NHL games fan telling me that I'm wrong). In Blizzard's case it's the opposite, they only have three franchises, but they milk them at their own pace, they are their own publisher as well, there's no deadline but their own (if there is one), they can and do push things back again and again to ensure that their games are above the quality "standards" of any other companies out there even including giants like Valve and BioWare (I'm not saying I hate Valve or BioWare, I'm just saying that especially recently they both took a different path then before, they seem to rush things a bit more now than they would have ever dared doing in the past).
In my opinion they really need to stay the way they are, whatever others say against them, or their business model, some of us... or rather many of us gamers might have the feeling that they are conservative because they haven't done anything "different" outside of their three franchises since their creation, and that they take their "sweet time" before getting things done, that's probably due to impatience as I mentioned, we all know that PC gamers are simply impatient overall, I would include myself in that group from time to time I can't hide it, but objectively speaking whatever we gamers think of Blizzard I really cross my fingers in hope that it never influences them ever, they've been great to the gaming market by providing good quality games over the years, and I wish that they keep doing it the same way, even if I myself am not interested at all in World of WarCraft for instance, I still can't deny that objectively speaking it is a good quality game, they've been milking it for years, but their milking seems to be good doesn't it? If people still play it today, if there's above 50 million subscribers worldwide it has to be mean something... the name Blizzard alone isn't fun enough for 50 million of gamers to stick together and keep playing, the actual game needs to be good, whatever we think of it, it works.
As I side note, I still don't agree with their recent business model "change" regarding StarCraft II and releasing three titles to tell the story of each of the three races for single-player campaigns, but that view of mine is simply a subjective judgment based on their fantastic revenues and how they seem so big that they could feed thousands of people all around the world and fund space shuttle missions by themselves because they literally could swim in their money, but their size, their influence and their business model still to this day haven't influenced the actual quality of their games in any negative way. I may not like what they're doing with SCII, but I do know inside me that SCII will sell like hotcakes for two principal reasons: 1) They are Blizzard, and still today it does mean that their games are of good quality and will be supported for years to come (just think of a decade-old game like Diablo II or even the original SC still being patched today, while most other developers out there outside of maybe Valve as a rare exception case usually leave their game for dead after two of three patches maximum) ... and 2) It's SC... so we are back at the main subject, being conservative, if their supporters and newcomers alike thought so then I don't think that SCII would be so hyped right now, the status of the SCII Beta, its popularity, the already good critical reception and reaction from the participants, etc., all points that Blizzard once again haven't disappointed, and even if there's anything "wrong" or "imbalanced" at release day then so what, it will be supported, patched, revised for the next years to come, look at how much the original SC and especially Diablo II have changed over the years, how many bugs were fixed, new items added, new features included and tweaked, new skills changed, new balance changes were done... no other companies than Blizzard, Valve and... well I can't think of any others at the moment, no others do it like that.
So, to conclude, no I don't think that Blizzard are conservative, I think that the gamers are simply impatient overall, but if Blizzard is indeed conservative for some reason I don't understand then I say so what? Let them be, their conservative approach to the gaming market hasn't killed them yet, still makes them pretty much the biggest developers out there with EA alongside (in terms of revenues, employees, etc.) and the fans don't seem to be tired of it, except perhaps for a minority, a minority that might well forget about it when they get their hands on and finally get to play either SCII or DIII for hours non-stop.