1, Entryism. Yeah, I know, we're the Lib Dems, who's going to bother? But the current system of conference reps does at least mean that someone who comes to conference with a voting pass has at least been given a cursory glance over by their local party. This could be mitigated by having a length of service clause (you can't vote till you've been a member for a given amount of time) but that wouldn't deter really determined entryists, and would mean that the one person you've thought of as a natural lib dem, who your local party has been courting for years, would also be denied a vote when under the current system they aren't. Also people who continually let their memberships lapse due to forgetfulness would be perpetually unable to vote. This could be mitigated by people signing up for direct debits.
2, Geographic concentration. This is already an issue - wherever conference is closest to supplies the majority of voting reps for that conference. I can't see OMOV making this any better, and I can see it potentially getting worse. A lot of policies we vote on have different applications in different regions. This could be mitigated by allowing online voting, but that opens up whole new vistas of cans of worms.
3, Tyranny of the Majority. Y'all just knew I was going to bring up John Stuart Mill at some point, didn't you? Dear old JS. If you have OMOV, and geographic concentration, and entryism, you run the risk of packing of policy votes. Now, arguably, this already happens. We've all** been in the hall for Julian and Evan's traditional "get rid of faith schools" motion/amendment, which it's quite clear the hall is going to vote for, and then the payroll vote come rolling in and vote it down. The payroll vote is smaller now, but that doesn't mean other packing factions won't emerge, and OMOV would make it lots easier for them. Packing of votes necessarily means smaller local parties/AOs/SAOs get less says, and I, for one, am in favour of diversity of opinion. This could be mitigated by retaining the current conference rep system.
4, Single Issue Pressure Groups. People would turn up en masse to vote on one motion. Can you imagine what 38 degrees would do to conference? This could be mitigated by retaining the current voting rep system, or by the long service requirement
5, Doesn't solve the problems it claims to solve. Becoming a conference rep is touted as a major barrier to participation in conference by proponents of OMOV. I have never known of a local party that does not have difficulty filling up all their available conference rep slots, even the ones that believe the emails that come from head office telling you you're entitled to less than you actually are. If turning up to your local party AGM and putting your hand up when the chair says "Who's going to conference, then?" is an insurmountable barrier to participation for a particular individual, I don't think that OMOV will make them more likely to participate. Maybe it will for a few, but not the majority. And yes, there IS a problem with moribund local parties in some areas, but OMOV doesn't suddenly invigorate them. No, the major thing that prevents people participating in conference is that it costs a small fortune, and again, OMOV does not solve this. This could be mitigated by not telling people a system is going to do something it demonstrably isn't and can't? IDK.
Now, I'm not actually dead set against OMOV. As I said at the beginning, it has a beguiling simplicity. But I would like to see genuine solutions to the problems I have with it before I vote for an unknown system over one that I know, and know works.
* up yours, Govey
** for a given definition of all